a case the in gear vibration will take place with less possibility therefor the reference ref may be made great to prevent catching false in gear vibrations according to another example the reference ref may be increased with respect to the increase of the roughness of the road the roughness of the road can be detected for example by using a vertical acceleration detector 31 fig8 mounted in a vehicle so that the up and down motion of the vehicle caused by bumps in the road can be detected by the vertical acceleration detector 31 the detected vertical acceleration of the vehicle such as shown in fig9 waveform a is added to an averaging circuit 32 for taking an average in as shown in block 32 thus averaging circuit 32 produces a signal representing the roughness of the road the signal produced from averaging circuit 32 is added to the reference ref thus the reference ref increases as the road becomes rough when the road becomes rough the vibration similar to the in gear vibration occurs thus by making the reference ref high such false in gear vibrations will not be caught instead the roughness of the road can be detected by taking a differential of the non drive wheel and then filtering the differential result the result can be processed in a similar manner described above referring to fig1 a modification of the flow chart is shown wherein steps 11 15 are illustrated to be replaced with steps 4 7 illustrated in fig3 in this modification two references ref1.