A corrected torque command value P_TRQ which is produced by correcting a torque command value TRQ with a torque command correcting unit 34 is substantially proportional to the armature current of an electric motor. Using the corrected torque command value P_TRQ and its average value P_ATRQ, an inferred temperature change .DELTA.tf of the electric motor is determined in each predetermined cycle time. The inferred temperature change .DELTA.tf calculated in a fuzzy inference operation, for example. The inferred temperature change .DELTA.tf is integrated by an integrating unit 37 to determine an accumulated temperature change .DELTA.Tf. The output of the electric motor is limited when the accumulated temperature change .DELTA.Tf exceeds a predetermined value.