To adjust a vehicle body direction of a towed vehicle along a movement direction by following after a traveling vehicle body traveling in a direction crossing a slope of inclined ground in the towed vehicle towed at a rear side of the traveling vehicle body.

A side slipping detection sensor 4 (4') is provided on the towed vehicle 2 (2'). A steering cylinder 22 (22') is operated by the output and a steering angle of a wheel 21 (21') of the towed vehicle 2 (2') is controlled. Thereby, a direction of a vehicle body direction axis 2A' in the traction vehicle 2 (2') is made coincident with an advancement direction U1. The side slipping detection sensor 4 (4') detects a direction of movement orbital which a point on the vehicle body of the towed vehicle 2 (2') draws on a ground and outputs deviation of the angle of the direction and the vehicle body direction axis 2A' (direction axis perpendicular to an axle). Thereby, the side slipping state of the towed vehicle 2 (2') is detected.