PURPOSE: To detect an obstruction with high accuracy by varying a scan area corresponding to the traveling state of a vehicle.

CONSTITUTION: This detector is provided with a controller 14 which varies the scan area by setting the scan angle of a stepping motor 17 larger as vehicle speed is increased and a steering angle is reduced. In such a way, the scan area of an obstruction sensor is varied corresponding to the vehicle speed or the steering angle. In other words, the scan area is varied corresponding to the traveling state of the vehicle by setting the scan angle so as to provide positive correlation for the vehicle speed and negative correlation for the steering angle. Thereby, the obstruction can be detected at high speed and surely.