Application of ion accelerator is one of important innovation for the society of radiology and radiation oncology. Electron linear accelerator has been utilized mainly for such application long time and became familiar to many hospitals. Sumitomo Heavy Industries, Ltd. (SHI) started manufacturing of accelerator from 1972 for Osaka University, Tohoku University, RIKEN separated sector cyclotron and so on. SHI also started manufacturing medical small cyclotron for PET tracer system in 1980. As for the cancer treatment application, SHI provided the first proton therapy system for National Cancer Center in Japan. The history of the developments and current activity is introduced.