To easily obtain signals for control synchronized with engine speed to control exciting force generating means for an active vibration control device.

For a rotation member 16 rotating at a rotation speed corresponding to an engine speed, a measuring plane 40 the distance L from the datum fixed on the engine 12-side varies continuously responding rotation angle of the rotation member 16 is provided. And by measuring the distance between the measuring plane 40 and the datum, a gap detecting means 44 outputting a signal corresponding to the distance between the opposing faces if realized.