Disclosed is a steam turbine power plant adapted to start operating very efficiently by highly accurate look-ahead control of a plurality of its startup constraints. The power plant includes a fundamental startup constraint prediction device 32 that calculates from a control input variable of the controller 12, 14 a prediction period about a fundamental startup constraint which is short in response time, a reference control input variables calculating device 33 that calculates such a reference control input variable of the controller 12, 14 as the value predicted and calculated by the fundamental startup constraint prediction device 32 will not exceed a limit value, other startup constraint prediction devices 35a, 35b each calculating a corresponding prediction period of data about desired one of other startup constraints from the prediction period of reference control input variables data, other control input variable calculating devices 36a, 36b each calculating corresponding other control input variables of the controller 12, 14 from the value predicted and calculated by the other startup constraint prediction device 35a, 35b, and a control signal output device 40, 41, 42 or 43 that outputs a command value to the controller 12, 14 in accordance with a value selected from the reference control input variable and the other control input variable.