In this paper, the authors propose a web-based VR system for operation training of medical therapy devices. Currently, their target is one of the radiation therapy devices called TrueBeam. Medical students must train as a radiation therapist if they want to be so. However, for such training, there are several problems, e.g., radiation therapy devices cannot be used so easily because they are very expensive and dangerous. Therefore, any training systems are needed, and then, the authors propose the web-based VR system in this paper. Although currently, their target is the radiation therapy device, the proposed web-based VR system can be used for operation training of other medical therapy devices because it has a very feasible system architecture based on recent Web technology. Therefore, the proposed web-based VR system has an aspect as a development framework for operation training systems of other medical therapy devices.