A ripple detecting device for detecting a ripple from an armature current outputted from a motor by overlapping a low band noise having a frequency of one/n (wherein “n” is a natural number more than 1) includes an initial ripple detecting device for detecting a cycle of the ripple in advance, a feature detecting device for detecting a feature of the ripple from the armature current outputted from the motor. The ripple detecting device further includes a ripple detecting control device for detecting the ripple by comparing the feature of the ripple with the low band noise having the frequency of one/n detected by the feature detecting device at a time before the present time with the feature of the ripple detected by the feature detecting device at the present time and judging that a ripple is generated when the features of the time before the present time and the present time present are resembled.