A steam turbine power plant (100; 101; 102) comprising: a heat source device configured to use a heat source medium to heat a low-temperature fluid and generate a high-temperature fluid; a steam generator configured to generate steam by the high-temperature fluid; a steam turbine that is driven by the steam; a power generator configured to convert driving force of the steam turbine into power; an adjuster (14; 15) configured to adjust a load of the plant; a measurer (11; 12; 13) configured to measure a state amount of the plant; a life consumption amount calculator configured to calculate life consumption amounts of a turbine rotor included in the steam turbine based on the state amounts measured by the measurer (11; 12; 13); a life consumption amount storage device configured to store the life consumption amounts of the turbine rotor; a thermal stress limit update timing determining device configured to determine a time when thermal stress limits of the turbine rotor are updated; characterized by comprising an accumulated life consumption amount calculator configured to accumulate, at a timing when the thermal stress limits of the turbine rotor are updated, life consumption amounts of the turbine rotor after the thermal stress limits are previously updated and calculate the accumulated life consumption amounts of the turbine rotor; a planned life consumption amount setting device (26; 126; 226) configured to set, based on the accumulated life consumption amounts of the turbine rotor, planned life consumption amounts (52; 55) of the turbine rotor for a time period to a time when the thermal stress limits are next updated; a thermal stress limit calculator configured to calculate and update the thermal stress limits based on the planned life consumption amounts (52; 55) of the turbine rotor; and a plant command value calculator configured to calculate, based on the thermal stress limits, a plant command value such that the plant command value does not exceed the thermal stress limits. a heat source device configured to use a heat source medium to heat a low-temperature fluid and generate a high-temperature fluid; a steam generator configured to generate steam by the high-temperature fluid; a steam turbine that is driven by the steam; a power generator configured to convert driving force of the steam turbine into power; an adjuster (14; 15) configured to adjust a load of the plant; a measurer (11; 12; 13) configured to measure a state amount of the plant; a life consumption amount calculator configured to calculate life consumption amounts of a turbine rotor included in the steam turbine based on the state amounts measured by the measurer (11; 12; 13); a life consumption amount storage device configured to store the life consumption amounts of the turbine rotor; a thermal stress limit update timing determining device configured to determine a time when thermal stress limits of the turbine rotor are updated; characterized by comprising an accumulated life consumption amount calculator configured to accumulate, at a timing when the thermal stress limits of the turbine rotor are updated, life consumption amounts of the turbine rotor after the thermal stress limits are previously updated and calculate the accumulated life consumption amounts of the turbine rotor; a planned life consumption amount setting device (26; 126; 226) configured to set, based on the accumulated life consumption amounts of the turbine rotor, planned life consumption amounts (52; 55) of the turbine rotor for a time period to a time when the thermal stress limits are next updated; a thermal stress limit calculator configured to calculate and update the thermal stress limits based on the planned life consumption amounts (52; 55) of the turbine rotor; and a plant command value calculator configured to calculate, based on the thermal stress limits, a plant command value such that the plant command value does not exceed the thermal stress limits.