To provide a detector for detecting catching/non-catching in a window glass highly precisely and easily detecting the catching from only a control side irrelevant to the performance of a motor side.

A duty signal of a prescribed duty ratio is applied to a power element 21 from a PWM(pulse width modulation) output part 14 so that a pulse current corresponding to the duty ratio is energized to a drive motor 19 via the power element 21. The pulse current is detected by a controller 11 via an operational amplifier 23 and an arithmetic part 12 of the controller 11 determines the catching/non-catching based on the change in the current value of the pulse. The controller 11, or the control side, heightens the frequency of the pulse current so that the number of pulses detected per unit hour increases to easily and highly precisely detect the catching.