An electric drive motor drives a door to move in an open/close direction. A pulse encoder outputs a series of pulses. Each pulse has a pulse duration sized to correspond to a rotation speed of the motor. The number of the pulses corresponds to the number of revolution of the motor. A door jamming detecting device for detecting a jamming of the door is configured to carry out calculating a total duration time (Tn) of a given number (n) of the series pulses outputted from the pulse encoder; detecting a given condition wherein the total duration time (Tn) is greater than a threshold value (Ts); and making a judgment of the door jamming when the given condition is kept for a given time (Tm).