To allow a trailer to come into a narrow passage in pushing and running the trailer in a towing tractor having a side shift device even when a handle of the towing tractor is not finely operated.

A coupler 10 connected to the trailer 20 is mounted to the side shift device 8 movably in the car body width direction, the trailer 20 is turned by moving the coupler 10 in the car body width direction (for example, the arrow C direction) and pushing the trailer 20 with the towing tractor 1, and thus the progressing direction of the trailer can be directly changed. Therefore, even an unskilled person can easily perform a turning operation in a predetermined direction, and the trailer 20 is made to come into even a narrow passage.