A steam turbine power plant, comprising: heat source equipment that heats a low-temperature flow by applying a heat medium and thus generates a high-temperature flow; a steam generator that generates steam using the high-temperature flow generated by the heat source equipment; a steam turbine driven by the steam generated by the steam generator; an electric generator that converts rotational motive power of the steam turbine into electric power; a controller that controls a load of the steam turbine power plant; a measuring instrument that measures a physical quantity of the plant; characterized in that a predictive value calculating device that calculates a predictive value of a startup constraint, estimated to occur during a fixed period of time following the current time of the steam turbine from a value measured by the measuring instrument; a first control input variable calculating device that calculates a first control input variable for the controller based upon the predictive value; a current value calculating device that calculates a current value of the startup constraint of the steam turbine based upon the value measured by the measuring instrument; a second control input variable calculating device that calculates a second control input variable for the controller based upon the current value; and a command output device (26, 27) configured so that while preferentially selecting the first control input variable, if the first control input variable is not calculated, the command output device (26, 27) selects the second control input variable instead, the command output device (26, 27) being further configured to output an appropriate command value to the controller according to the kind of selected control input variable. heat source equipment that heats a low-temperature flow by applying a heat medium and thus generates a high-temperature flow; a steam generator that generates steam using the high-temperature flow generated by the heat source equipment; a steam turbine driven by the steam generated by the steam generator; an electric generator that converts rotational motive power of the steam turbine into electric power; a controller that controls a load of the steam turbine power plant; a measuring instrument that measures a physical quantity of the plant; characterized in that a predictive value calculating device that calculates a predictive value of a startup constraint, estimated to occur during a fixed period of time following the current time of the steam turbine from a value measured by the measuring instrument; a first control input variable calculating device that calculates a first control input variable for the controller based upon the predictive value; a current value calculating device that calculates a current value of the startup constraint of the steam turbine based upon the value measured by the measuring instrument; a second control input variable calculating device that calculates a second control input variable for the controller based upon the current value; and a command output device (26, 27) configured so that while preferentially selecting the first control input variable, if the first control input variable is not calculated, the command output device (26, 27) selects the second control input variable instead, the command output device (26, 27) being further configured to output an appropriate command value to the controller according to the kind of selected control input variable.