A gas turbine combined cycle plant includes a fuel gas heater configured to heat fuel gas to be supplied to a combustor of a gas turbine using heated water heated by an exhaust heat recovery boiler; a return valve disposed on a heated water returning line to return heated water passing through the heater to the boiler; a dump valve disposed on a condensate line bifurcated from the heated water returning line between the return valve and the heater to return the heated water to a condenser; and a control unit configured to control operation of the return and dump valves. When load of the turbine falls below a lower limit, the control unit closes the return valve and keeps the opening degree of the dump valve at a specified degree to reduce a flow rate of the heated water flowing in the heater for a predetermined time.