PURPOSE: To enhance the reliability of a rotating electric machine by determining the temperature difference between more than one temperature measured for each stator and producing an alarm based on the difference between the temperature difference and a standard temperature difference thereby preventing troubles caused by insufficient cooling of the stator coil.

CONSTITUTION: Temperature measuring elements 21 provided for every plurality of stator coils detect the refrigerant temperatures T1-Tn at the outlets of refrigerant channels, and a current transformer 27 detects an armature current Iph. Each detection signal is read into a RAM in an operating unit 29 through a converting element 25 and a converter 28. The operating unit 29 compares the temperature difference with a standard temperature difference under normal operating condition being determined from the operating data in the past. If the difference deviates from an alarm level, a signal is delivered to an alarm 23 which notifies an operator that the stator coil is abnormal.