When an air pump is operated, a brush temperature increase rate stored in an ECU in relation to a discharge pressure of the air pump and a brush temperature is multiplied by a calculation cycle time, and the resultant value is cumulatively added. When the air pump is stopped, a brush temperature decrease rate stored in the ECU in relation to the brush temperature is multiplied by the calculation cycle time, and the resultant value is cumulatively added. Each of the cumulative values read from a map is added to and subtracted from an initial brush temperature value, respectively so as to estimate the brush temperature.