[en] ''Yayoi'' is the nickname for the fast neutron reactor of the University of Tokyo and reached to the critical range in April, 1970 for the first time. It is an air cooled type fast reactor for condensed uranium installed in Tokai Village, Ibaraki Prefecture with thermal powers of 2 Kw (steady working), 200 Kw (unsteady working) or 1 Gw (reactive pulse working). There were submitted to explain about summaries of irradiation equipment and medical irradiation field, and technical capacity of irradiation treatment by ''Yayoi''. Additional data were also prepared explaining the purpose of medical treatment, expected dose, counterplans for security, minimization of general exposed dose radiation control and in emergency cases. (Kobatake, H.)