The Challenge in Developing Monex, Inc.’s Open API

Introduction: What is Open API?

Open API developed by Monex, Inc. is a technology that securely connects our customers’ everyday external services with Monex’s systems, offering a more seamless and convenient financial experience. For example, when you register your Monex account with a household budgeting app, your asset balances and holdings update automatically within the app, allowing you to see your financial status at a glance, anytime, anywhere. Through this mechanism, finance becomes something more accessible and familiar in everyday life. But the reason Monex embarked on this development went far beyond technological innovation. It was a bold challenge to embody a cultural shift where we “create our own future.” This time, we spoke with Daisuke Houki, manager of the Product Delivery Group 1, System Development Department 1, who led the Open API initiative and guided the journey from one small realization to an organization-wide transformation.

A Turning Point Born from a Realization — The Beginning of “Connecting Ideas”

― First, what led you to believe Monex, Inc. should develop its own Open API?
 

Houki: When I joined Monex in 2015, I was involved in the in-house development of our core securities system. At that time, most firms in the securities industry either purchased external systems or outsourced their development.

But Monex decided to take a different path by building our core system ourselves. This decision represented not just a technical choice, but a shift in mindset: a move toward a culture of building our own future with our own hands.

By 2017, our core system migration was mostly complete, and we entered a new phase—“what should we create next?”

Back then, new services were emerging one after another and even fundamental functions, such as order processing and trading, were being developed separately for each service. That duplication created inefficiencies and slowed down innovation.

Then, in conversations with external tech firms, I repeatedly heard, “System integration should be done through open APIs,” as if it were the most natural thing in the world. I was shocked to realize that in other industries, API integration was already the norm. We even missed potential business partnerships because our open API capabilities weren’t mature enough. I thought to myself: “We’ll be left behind if we don’t change.” So, in April 2017, I proposed starting discussions on building a common API platform across the company.

(Daisuke Houki, manager, Product Delivery Group 1, System Development Department 1, Monex, Inc.)

 

Moving the Company Forward Together — Cultivating the Soil for Co-Creation
 

― Was the idea of an Open API easily understood within the company?

Houki: Since APIs are an invisible technology, it was initially difficult to communicate their value—“What’s in it for us?”

So, we began by deepening understanding internally. We invited external experts to give study sessions, shared insights via internal chats, and even involved the then head of Corporate Planning.

At our company, we have a flat, open culture where ideas can be discussed freely across departments. This atmosphere made it easy for new ideas like Open APIs to spread naturally, and soon, understanding grew throughout the organization.

After about a year of study sessions and cross-department discussions, a shared conviction emerged:
“This is something we should challenge ourselves to achieve.”
That shared belief ultimately led to budget approval and the start of the project.
 

― What was most important to you as you advanced the development?
 

Houki: The key was to learn the right knowledge from the right people. Open API would have no value if it didn’t comply with international standards. So we started from the fundamentals, learning directly from specialists and incorporating regular external reviews. With expert advice and validation, we ensured our design aligned with global standards and introduced truly innovative technology in the best possible way.
 

― What was the first Open API you built?

Houki: Our first creation was a read-only API that allowed external services to securely view account information. We began by linking with the household budgeting app Money Forward, enabling users to automatically check their Monex account balances and transaction histories within the app.

Customers could now monitor their daily financial status without logging into Monex, Inc.’s website—an innovation that, at the time, very few securities companies had achieved. It was a pioneering step in the industry.

 

Expanding Connectivity — A New Strategy Shaped by Open APIs

― Once the initial API was implemented, how did integration evolve?
 

Houki: Once we saw the effectiveness of the Open API both inside and outside the company, we developed more types of APIs. One of the most important was the order placement API, first for equities, then for mutual funds. These APIs became the foundation for new internal services, including ferci, our smartphone investment app for younger investors that allows stock purchases starting from a single share, as well as other apps supporting U.S. stock trading.

This shared API platform greatly improved our development speed and efficiency, evolving Monex’s core philosophy of “think for ourselves, build for ourselves” to the next level.
 

― So, this framework became the strength that enables quick responses to customers’ diverse investment styles.
 

Houki: Exactly. Starting with Money Forward, our partnerships expanded to include AEON Bank and NTT Docomo. For AEON Bank, we developed a dedicated API to enhance accessibility while ensuring security. We are also considering a sweep function that will automatically transfer funds in the future.

The d Payment mini app launched by NTT Docomo in July 2025 was also implemented using our API platform. This technology will continue to serve as the bridge connecting services between companies as we strengthen our collaboration. These partnerships were made possible by the robust security of our Open API platform. We adopted the OAuth 2.0^* framework, which allows users to grant access permissions securely without sharing their passwords with external services, significantly reducing the risk of data leaks or unauthorized use.

Recently, we’ve also added passkey authentication, enabling a safer, smoother, and more user-friendly experience.
(Note: OAuth 2.0 is an authorization framework that enables users to grant access securely without sharing their passwords with external services.)

Learning and Creating Together — The Philosophy of the API Economy

― As a development philosophy, you emphasize co-creation with external partners.
 

Houki: Yes, co-creation is at the heart of our philosophy. As more companies open their APIs, each one no longer needs to handle every part of the customer experience alone.

Just as Uber uses Google Maps for navigation and Stripe for payments, we believe that new value is created when each company’s strengths are combined through APIs. We learn from experts, collaborate with strong partners, and grow together. That approach—the willingness to learn and co-create—has been the driving force behind our technological evolution. Our Open API was itself a product of co-creation, born from constant dialogue with external experts and partner companies.

Connecting Technology, Opening the Future

― What are your prospects for the future?
 

Houki: I believe the next key will be the fusion of AI and APIs. Very soon, you’ll be able to ask an AI like ChatGPT or Gemini, “Tell me about my financial situation,” and behind the scenes, APIs will securely gather the information and respond with precision. We already see services overseas that combine AI with order functions and I expect this to become common in Japan as well. At Monex, Inc., we’re developing APIs that AI can utilize, aiming to make financial experiences more natural, intuitive, and human-centric. To make that future real, Monex, Inc. will continue to pursue co-creation—constantly imagining, innovating, and delivering new value to our customers.

In Closing
 

Open API technology has become a core foundation for Monex, Inc., nurturing our culture of co-creation and supporting the creation of new partnerships and customer experiences.
Through technology, we are making finance clearer, simpler, and more open to empower every individual to take ownership of his or her financial life.

As we move forward, Monex, Inc. will continue to work with our customers as we look ahead and open new doors in the future of finance.

November 14, 2025
Employees’ department affiliation, title, and comments are current as of the time of the interview.