In an operation method of a multi-shaft combined cycle plant, a low-load mode in which an output of the multi-shaft combined cycle plant is adjusted by adjustment of only an output of a gas turbine and a high-load mode in which the output of the multi-shaft combined cycle plant is adjustable by adjustment of the output of the gas turbine and adjustment of an output of a steam turbine are switched according to a demanded load. In the low-load mode, steam at a standby flow rate at which the steam turbine is capable of maintaining a predetermined initial load is supplied to the steam turbine, and the initial load is applied to the steam turbine.