PURPOSE: To monitor a coil temperature with an excellent accuracy and reliability and with an excellent economic quality in the whole operational region of a rotating electric machine.

CONSTITUTION: The coil or load current of a rotating machine is sensed by a current sensor. By taking it into a first computing element 21 every sampling time for monitoring, the average value of the coil or load current is calculated. Based on the average value, a saturated rise value of a coil temperature of the rotating machine is calculated by a second computing element 22. Also, the coil temperature and a cooling air temperature are sensed by a coil temperature sensor 17 and a cooling air temperature sensor 18 respectively. Then, based on them, the rise value of the coil temperature and the changing rate thereof are calculated by a third computing element 23. Further, based on the output signals of the second and third computing elements 22, 23, monitoring levels are set sequentially by fourth and fifth computing elements 24, 25 according to a predetermined estimated rise value of the coil temperature and a coil time constant. Subsequently, by comparators 26, 28, the predictive rise value of the coil temperature and the predictive changing rate thereof at the next monitoring time are compared with the real value and the real rate at the present monitoring time respectively. Thereby, the existence of an abnormality is decided.