Disclosed is a steam turbine power plant adapted to start operating safely even if prediction accuracy of its startup constraints cannot be obtained. The system calculates predictive values and current values of startup constraints of a steam turbine from process variables of plant physical quantities, next calculates in parallel both a first control input variable for a heat medium flow controller based on predictive values, and a second control input variable for a main steam control valve based on the current values, and while preferentially selecting the first control input variable, if the first control input variable is not calculated, selects the second control input variable instead. After the selection of at least one of the first and second control input variables, the system outputs an appropriate command value to the heat medium flow controller and the main steam control valve according to the kind of selected control input variable.