A single shaft combined cycle power plant comprising: a gas turbine that is driven by combustion gas and discharges exhaust gas; an exhaust heat recovery boiler that uses the exhaust gas from the gas turbine as a heat source and includes a high-pressure drum for generating high-pressure steam and a low-pressure drum for generating low-pressure steam having a pressure lower than that of the high-pressure steam; a high-pressure steam turbine that is coaxially arranged with the gas turbine and driven by the high-pressure steam fed from the high-pressure drum via a high-pressure steam control valve; a low-pressure steam turbine that is coaxially arranged with the high-pressure steam turbine; a power generator that is coaxially arranged with the low-pressure steam turbine; a low-pressure turbine steam supply pipe that has a low-pressure main steam control valve and feeds the low-pressure steam from the low-pressure drum to the low-pressure steam turbine via the low-pressure main steam control valve; a start-up boiler that generates auxiliary steam; a start-up time auxiliary steam supply pipe that connects the start-up boiler with the low-pressure turbine steam supply pipe at upstream side relative to the low-pressure main steam control valve and feeds auxiliary steam at start-up time; an auxiliary steam flow control valve that controls flow rate of steam fed from the start-up time auxiliary steam supply pipe to the low-pressure turbine steam supply pipe; and a controller that feeds auxiliary steam from the start-up boiler to the low-pressure steam turbine via the low-pressure turbine steam supply pipe by controlling valve opening degree of the auxiliary steam flow control valve to allow the low-pressure steam turbine to generate a drive force to thereby perform speed-up control in a unified manner for the gas turbine, the high-pressure steam turbine, the low-pressure steam turbine, and the power generator during a time period from when the gas turbine is started up to when rotation speed of the gas turbine reaches a rotation speed allowing self-sustained operation using the combustion gas. a gas turbine that is driven by combustion gas and discharges exhaust gas; an exhaust heat recovery boiler that uses the exhaust gas from the gas turbine as a heat source and includes a high-pressure drum for generating high-pressure steam and a low-pressure drum for generating low-pressure steam having a pressure lower than that of the high-pressure steam; a high-pressure steam turbine that is coaxially arranged with the gas turbine and driven by the high-pressure steam fed from the high-pressure drum via a high-pressure steam control valve; a low-pressure steam turbine that is coaxially arranged with the high-pressure steam turbine; a power generator that is coaxially arranged with the low-pressure steam turbine; a low-pressure turbine steam supply pipe that has a low-pressure main steam control valve and feeds the low-pressure steam from the low-pressure drum to the low-pressure steam turbine via the low-pressure main steam control valve; a start-up boiler that generates auxiliary steam; a start-up time auxiliary steam supply pipe that connects the start-up boiler with the low-pressure turbine steam supply pipe at upstream side relative to the low-pressure main steam control valve and feeds auxiliary steam at start-up time; an auxiliary steam flow control valve that controls flow rate of steam fed from the start-up time auxiliary steam supply pipe to the low-pressure turbine steam supply pipe; and a controller that feeds auxiliary steam from the start-up boiler to the low-pressure steam turbine via the low-pressure turbine steam supply pipe by controlling valve opening degree of the auxiliary steam flow control valve to allow the low-pressure steam turbine to generate a drive force to thereby perform speed-up control in a unified manner for the gas turbine, the high-pressure steam turbine, the low-pressure steam turbine, and the power generator during a time period from when the gas turbine is started up to when rotation speed of the gas turbine reaches a rotation speed allowing self-sustained operation using the combustion gas.