PURPOSE: To increase the load capacity and to enhance the braking force as necessary by incorporating a trailer part in an automobile without changing its size.

CONSTITUTION: An attaching ring 4 is connected to the rear part of a differential device 3 in a trailer vehicle 1, and a board 5 is coupled to the attaching ring 4. A rail 9 is attached to the board 5 through the intermediary of a pair of removable boards 6, and a holding member 12 having a roller 10 which can roll along the rail 9 is removably provided to a wheel attaching stay 13. A wheel 16 is rotatably journalled to the wheel attaching stay 13, and a trailer load carrying platform 19 is fitted in a hole 23 formed in a platform floor 22 of an automobile through the intermediary of a suspension composed of a cylinder 17 and a piston rod 18.