A steam temperature control device for controlling a main steam temperature which is a temperature of main steam generated from a superheater of a boiler , wherein the boiler including a desuperheater which decreases a temperature of steam using an amount of spray water based on a water amount control value for controlling a degree of opening of a spray water supply valve , the boiler further including a superheater for heating steam discharged from the desuperheater by an exhaust gas discharged from a furnace, the steam temperature control device comprising: a water amount control value calculation part which calculates the water amount control value based on the main steam temperature such that the main steam temperature becomes a target temperature, and transmits the water amount control value to the spray water supply valve , the water amount control value calculation part is configured to transmit, as a lower limit value of the water amount control value, a water amount control minimum value to the spray water supply valve , characterised in that the water amount control minimum value is calculated based on a moving average of a detected value of a supply water flow rate detector during a fixed period or a moving average of the water amount control value during a fixed period, wherein the supply water flow rate detector detects an instantaneous value of spray water supplied to the desuperheater , the water amount control value calculation part transmits a value to be calculated by multiplying the moving average or a value based on the moving average by a main steam temperature coefficient to the spray water supply valve as the water amount control minimum value, the main steam temperature coefficient indicates the lowest value when the main steam temperature is equal to or below the set temperature, and is decided based on the main steam temperature when the main steam temperature is higher than the set value. a water amount control value calculation part which calculates the water amount control value based on the main steam temperature such that the main steam temperature becomes a target temperature, and transmits the water amount control value to the spray water supply valve , the water amount control value calculation part is configured to transmit, as a lower limit value of the water amount control value, a water amount control minimum value to the spray water supply valve , characterised in that the water amount control minimum value is calculated based on a moving average of a detected value of a supply water flow rate detector during a fixed period or a moving average of the water amount control value during a fixed period, wherein the supply water flow rate detector detects an instantaneous value of spray water supplied to the desuperheater , the water amount control value calculation part transmits a value to be calculated by multiplying the moving average or a value based on the moving average by a main steam temperature coefficient to the spray water supply valve as the water amount control minimum value, the main steam temperature coefficient indicates the lowest value when the main steam temperature is equal to or below the set temperature, and is decided based on the main steam temperature when the main steam temperature is higher than the set value. <EOS>