[en] A series of the works recently conducted for safety evaluation of the experimental fast reactor ''JOYO'' gave favorable results for further power increase from the previously authorized power of 50 MWt. Major technical topics associated with the recently conducted safety evaluation are discussed. Safety examination given by the regulatory authority for increasing reactor power to two levels (75 MWt for Mark-I core and 100 MWt for Mark-II core) was carried out for about one year from September 1977 and approval by the regulatory authority was granted on 20 September 1978. 2 refs