To avoid an erroneous detection of a noise, etc., as the abnormality of a current sensor, etc., even if the revolution of an AC motor which is a control object fluctuates over a wide range.

Times t1 and t2 when the sum S of the phase current detection values of respective phases is out of its allowable range in the period of electrical angle or the sum (t) of the times t1 and t2 in the period of electrical angle T of a motor is counted and the occupation ration of the time (t)=t1+t2 in the period T is judged in comparison with a threshold. If t/T or (t1+t2)/T is relatively large, it is assumed that the abnormality of a current sensor, etc., really exists and, if this stage exists continuously over the (n)th electrical angle period, it is finally judged that the abnormality of the current sensor, etc., exists. To the contrary, if t/T or (t1+t2)/T is relatively small, it is assumed that the phenomenon is not the abnormality of the current sensor, etc., but a noise.