[en] The experimental fast breeder reactor ''Joyo'' was constructed in Oarai Engineering Center, Power Reactor and Nuclear Fuel Development Corporation, and it is the loop type reactor cooled with sodium. The thermal output was planned to be 100 MWt, but for the time being, it is operated at 50 MWt. Two objects of the construction of Joyo are the accumulation of experience through its design, construction and operation for the development of FBRs and the use as the irradiation facility for fuel and materials for FBR development. The main machinery and equipments were manufactured and installed with domestic technologies. The integrated functional test was started on January 10, 1975, and completed on March 8, 1977. The planning of the test, the organization for the test, and the facilities and equipments for the test are described. Fuji Electric Co. was in charge of the tests on fuel handling equipment, fuel accepting and storing facility, maintenance equipment, and electric installations. Fuel exchanger address confirmation test, fuel transport simulation test, reactor protecting function test, integrated washing test, the exchange of waste gas prefilters in waste gas treatment system, and the analysis of adhering sodium are explained briefly. Joyo attained the criticality on April 24, 1977, and low output operation is carried out. (Kako, I.)