The authors performed a usability improvement study for a shoulder continuous passive motion (CPM) rehabilitation device based on the usability engineering process of IEC 62366, a mandatory standard for the development of medical devices. To enhance the usability of the entire development process for a shoulder CPM device, the authors 1) performed user research to determine design requirements and 2) evaluated the usability of the device. Requirements for a shoulder CPM device were derived through rehabilitation device comparisons, functional analysis, context inquiry and observation, and interviews. The authors used expert reviews and comparison usability evaluation methods for shoulder CPM prototyping. The methods and techniques of these design researches were declared in IEC 62366, but IEC 62366 does not include any guideline in detail. The results of this study can be used to guide the development of a user interface that meets the level of usability standards required for medical devices.