This paper treats operation training systems for medical therapy devices. Medical therapy devices cannot be used so easily because they are very expensive and dangerous. Therefore, any training systems are needed, and then, the authors have already proposed a web-based VR system for operation training of medical therapy devices. Their target is one of the radiation therapy devices called TrueBeam, and currently for displaying 3D models of TrueBeam and a patient, a standard display monitor is used. As a display device of the proposed web-based VR system, several commercial products dedicated for VR or AR applications are available, those are MS Hololens, Epson BT-300, Vox+ Z3 and so on. However, to use them practically, there are some problems, e.g., some of such VR/AR devices are too heavy, some have a too narrow display area, which affects the training efficiency. Therefore, the authors investigated the usefulness of several VR/AR devices as a display device of their proposed web-based VR system, and this paper reports comparative experimental results.