A cooler includes an estimator. The estimator is configured to estimate a coolant temperature from a measurement value of the temperature sensor. The estimator is configured to determine a difference by subtracting an immediately preceding measurement value from a current measurement value, determine a correction value from the difference, and output an estimated value of the coolant temperature. The estimated value is obtained by adding the correction value to the current measurement value. The correction value is obtained by multiplying a gain by the difference. The gain is determined from a time constant, the gain is determined according to a flow rate of the coolant. The time constant is obtained when a transfer function of heat transferred from the coolant to the temperature sensor is modeled as a first order lag system.