An activation control device for a steam turbine plant, the steam turbine plant including a heat source device for heating 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, and a steam turbine driven by the steam, the activation control device comprising: a parameter acquisition circuit for acquiring an initial value of a state amount parameter of the steam turbine plant; a storage circuit for storing a correlation between the initial value of the state amount parameter and a plant operation amount that includes a control reference value related to a control target amount and time lengths of phases from the start to the completion of the activation of the steam turbine plant, and characterized by an operation amount determination circuit for determining time lengths of the phases and a control reference value based on the initial value of the state amount parameter, the initial value being acquired by the initial state parameter acquisition circuit, and based on the correlation stored in the storage circuit, wherein the initial value is a time elapsed after a stop of the steam turbine plant, and the time lengths of the phases continuously change with respect to the time elapsed after the stop of the steam turbine plant; an activation schedule generation circuit for generating, for the control target amount, activation schedules of the phases based on the time lengths of the phases and the control reference value, the time lengths and the control reference value being determined by the operation amount determination circuit, the activation schedule generation circuit further generating an activation schedule for a time period from the start to the completion of the activation of the steam turbine plant by combining the activation schedules for the phases; and an activation control circuit configured to generate a command value for the steam turbine plant in accordance with the activation schedule generated by the activation schedule generation circuit and output the generated command value to the steam turbine plant. a heat source device for heating 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, and a steam turbine driven by the steam, a parameter acquisition circuit for acquiring an initial value of a state amount parameter of the steam turbine plant; a storage circuit for storing a correlation between the initial value of the state amount parameter and a plant operation amount that includes a control reference value related to a control target amount and time lengths of phases from the start to the completion of the activation of the steam turbine plant, and characterized by an operation amount determination circuit for determining time lengths of the phases and a control reference value based on the initial value of the state amount parameter, the initial value being acquired by the initial state parameter acquisition circuit, and based on the correlation stored in the storage circuit, wherein the initial value is a time elapsed after a stop of the steam turbine plant, and the time lengths of the phases continuously change with respect to the time elapsed after the stop of the steam turbine plant; an activation schedule generation circuit for generating, for the control target amount, activation schedules of the phases based on the time lengths of the phases and the control reference value, the time lengths and the control reference value being determined by the operation amount determination circuit, the activation schedule generation circuit further generating an activation schedule for a time period from the start to the completion of the activation of the steam turbine plant by combining the activation schedules for the phases; and an activation control circuit configured to generate a command value for the steam turbine plant in accordance with the activation schedule generated by the activation schedule generation circuit and output the generated command value to the steam turbine plant.