[en] 'Joyo', as the first FBR (Fast Breeder Reactor) in Japan, was born at Oarai, Ibaraki prefecture, in 1977. History of Joyo is described. The part, design, safety review, agreement, construction, total functions test, the first critical state and operation are explained. Outline of facilities, main changes of designs, improvement construction, past records of operation and maintenance are described. MK-III plan has been proceeding since 1987 in order to improve its performance to obtain the largest fast neutron flux in the world. Increasing irradiation performance was investigated. (S.Y.)