The concept of MyData emerged as a paradigm shift in personal data management and the process of seeking to transform the current organization-centered system. MyData enables the utilization of one's own personal information that is scattered among various institutions as a system for data subjects to exercise rights of self-determination. We aimed to develop and demonstrate a MyData platform (MyHealthData) that allows data subjects to download and manage health-related personal data stored in various medical institutions. The platform consists of a mobile app for users, API (application-program interface) for data conversion and exchange installed in the hospital information system (HIS), and a relay server connected to the blockchain to ensure data integrity. User surveys were conducted to explore perceived usefulness, perceived ease of use, and satisfaction. We provided four services to users through the platform developed in this study: inquiring about medical and health checkup records, health coaching, checking conditions of participation in clinical research, and claims, all by using an app. A total of 1228 participants signed up for the service and the overall user satisfaction was high, especially with 'inquire about medical and health checkup records'. MyData brings a user-centered paradigm in which data subjects can directly participate in the use of their own data. MyData will improve healthcare data interoperability, allowing it to be used not only in research areas but also in other areas by sharing and integrating various healthcare data.