When it is determined that the number of times that motor rotation-indicative quantity has been detected has exceeded a measurement interval, a feedback speed value is calculated from a motor rotation-indicative quantity detected immediately before, a motor rotation-indicative quantity detected immediately before the last time a measurement interval determination means determined that the number of times had exceeded the measurement interval, and time that has elapsed since the last time the measurement interval determination means determined that the number of times had exceeded the measurement interval, and the measurement interval having a length calculated from a greater one between the calculated feedback speed value and a speed command value is set for the measurement interval determination means to refer to.