A gas turbine combined cycle (GTCC) power generation plant (100) equipped with a control unit which performs a load reduction following operation with respect to a fuel adjustment valve (Vd), a main steam valve (V1), and a bypass valve (V4), wherein, when a load reduction request for reducing a GTCC load target value has been input in a closed bypass operation, the degree of opening of the fuel adjustment valve (Vd) is reduced in accordance with the target value while the main steam valve (V1) is in an open state, and the bypass valve (V4) is placed in an open state, after which the bypass valve (V4) is placed in the closed state when the GTCC load reaches the target value.