An electronic control unit (9) assists parking of a host vehicle into a parking space. Ultrasonic sensors (6, 8) detect the shape of the side edge of a road and the presence of an obstacle at the road edge. The electronic control unit (9) sets one corner of the obstacle positioned on the side opposite to the road side edge as an originating corner, sets an imaginary line that follows the shape of the road side edge and extends from the originating corner, and recognizes the space between the imaginary line and the road side edge as a parking space. The electronic control unit (9) then determines whether the vehicle can be parked in the recognized parking space.