An activation control device for a steam turbine plant, the steam turbine plant including a heat source device configured to heat a low-temperature fluid using a heat source medium to generate a high-temperature fluid, a steam generator for generating steam by thermal exchange with the high-temperature fluid, a steam turbine to be driven by the steam, and an adjuster for adjusting a plant operation amount, the activation control device comprises: a predicting unit for calculating a predicted value for at least one constraint to be used to control the activation of the steam turbine; an activation control parameter setting unit configured to calculate, based on an initial value of a plant state amount, an activation control parameter, which is a coefficient included in a function to set an activation schedule, to be used to control the activation of the steam turbine; and a plant operation amount calculator configured to determine the plant operation amount, based on the predicted value for a constraint, the predicted value being calculated by the predicting unit, and on the activation control parameter calculated by the activation control parameter setting unit, so that the constraint does not exceed a predetermined limit characterized in that the predicting unit further includes a plant state amount prediction calculation circuit for calculating a predicted future state amount of the plant based on the plant operation amount and the plant state amount and a constraint prediction calculation circuit (25, 26, 27) for calculating a predicted value of the constraint based on the predicted future state amount of the plant calculated by the plant state amount prediction calculation circuit, and in that the plant operation amount calculator includes a requested operation amount calculation circuit (28, 29, 30) for calculating a requested operation amount of the plant based on the predicted value of the constraint, the predicted value being calculated by the constraint prediction calculation circuit (25, 26, 27), and on the activation control parameter calculated by the activation control parameter setting unit so that the constraint does not exceed the predetermined limit, and a low value selector for selecting the minimum requested operation amount of the plant among the requested operation amounts of the plant calculated by the requested operation amount calculation circuit. a heat source device configured to heat a low-temperature fluid using a heat source medium to generate a high-temperature fluid, a steam generator for generating steam by thermal exchange with the high-temperature fluid, a steam turbine to be driven by the steam, and an adjuster for adjusting a plant operation amount, a predicting unit for calculating a predicted value for at least one constraint to be used to control the activation of the steam turbine; an activation control parameter setting unit configured to calculate, based on an initial value of a plant state amount, an activation control parameter, which is a coefficient included in a function to set an activation schedule, to be used to control the activation of the steam turbine; and a plant operation amount calculator configured to determine the plant operation amount, based on the predicted value for a constraint, the predicted value being calculated by the predicting unit, and on the activation control parameter calculated by the activation control parameter setting unit, so that the constraint does not exceed a predetermined limit characterized in that the predicting unit further includes a plant state amount prediction calculation circuit for calculating a predicted future state amount of the plant based on the plant operation amount and the plant state amount and a constraint prediction calculation circuit (25, 26, 27) for calculating a predicted value of the constraint based on the predicted future state amount of the plant calculated by the plant state amount prediction calculation circuit, and in that the plant operation amount calculator includes a requested operation amount calculation circuit (28, 29, 30) for calculating a requested operation amount of the plant based on the predicted value of the constraint, the predicted value being calculated by the constraint prediction calculation circuit (25, 26, 27), and on the activation control parameter calculated by the activation control parameter setting unit so that the constraint does not exceed the predetermined limit, and a low value selector for selecting the minimum requested operation amount of the plant among the requested operation amounts of the plant calculated by the requested operation amount calculation circuit.