An operation method of a multi-shaft combined cycle plant including a gas turbine, an exhaust heat recovery boiler that generates steam using an exhaust gas from the gas turbine, and a steam turbine that is driven by the steam generated by the exhaust heat recovery boiler, in which a gas turbine rotor of the gas turbine and a steam turbine rotor of the steam turbine are not mechanically connected to each other, the method comprising: switching, according to a demanded load, between a low-load mode in which an output of the multi-shaft combined cycle plant is adjusted by adjustment of only an output of the 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 the steam turbine; and supplying steam at a standby flow rate at which the steam turbine is capable of maintaining a predetermined initial load to the steam turbine, and applying the initial load to the steam turbine even in the low-load mode, wherein when the demanded load becomes low in the high-load mode and the high-load mode is switched to the low-load mode, the steam at the standby flow rate is supplied to the steam turbine, wherein in a case where a load change time point which is a time point at which the demanded load after the switching to the low-load mode becomes a demanded load which is a high load that is not to be handled by adjustment of only the output of the gas turbine is recognized in advance, the steam at the standby flow rate is supplied to the steam turbine from the switching to the low-load mode to a steam supply stop time point which is a time point prior to the load change time point, the supply of the steam to the steam turbine is stopped from the steam supply stop time point to the load change time point, and the multi-shaft combined cycle plant transitions to the high-load mode from the load change time point, and wherein the steam supply stop time point is a time point at which an internal temperature of the steam turbine is equal to or higher than the predetermined temperature when the load change time point is reached. switching, according to a demanded load, between a low-load mode in which an output of the multi-shaft combined cycle plant is adjusted by adjustment of only an output of the 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 the steam turbine; and supplying steam at a standby flow rate at which the steam turbine is capable of maintaining a predetermined initial load to the steam turbine, and applying the initial load to the steam turbine even in the low-load mode, wherein when the demanded load becomes low in the high-load mode and the high-load mode is switched to the low-load mode, the steam at the standby flow rate is supplied to the steam turbine, wherein in a case where a load change time point which is a time point at which the demanded load after the switching to the low-load mode becomes a demanded load which is a high load that is not to be handled by adjustment of only the output of the gas turbine is recognized in advance, the steam at the standby flow rate is supplied to the steam turbine from the switching to the low-load mode to a steam supply stop time point which is a time point prior to the load change time point, the supply of the steam to the steam turbine is stopped from the steam supply stop time point to the load change time point, and the multi-shaft combined cycle plant transitions to the high-load mode from the load change time point, and wherein the steam supply stop time point is a time point at which an internal temperature of the steam turbine is equal to or higher than the predetermined temperature when the load change time point is reached.