PURPOSE: To prevent a drawn wheel from bend relative to a tractive wheel in the backing of a vehicle body by providing a lock fitting changed over between the working position and non-working position on couplers in either one of couplers.

CONSTITUTION: A vehicle body frame 1 of a compact motor car A is provided on the rear part with a hook type coupler C. A vehicle body frame 9 of a trailer B is provided on the front part with a connected coupler C' provided with an annular part 10a connected to the hook type coupler C and an arm part 10b fixed to the vehicle body frame 9. Further, the coupler C of the compact motor car A is provided with a lock fitting 11 constituted to be changed over between the working position for blocking a trailer B from the left and right swing relative to the compact motor car A and the non-working position about the lateral axis X.