To simplify structure, to reduce an angle error, and to dispense with large-scaled modification for a trailer.

Favorable image positions of corner edges 2R, 2L of a front panel 2a or front vertical markers 5aR, 5aL provided in corner edges, and side face vertical markers 5bR, 5bL, which are photographed by a TV camera mounted on a tractor, are detected to find a connection angle and a curved direction of the trailer based on the image positions of the corner edges or the vertical markers approaching to the camera.