To easily connect a tractor to a trailer even if a rubber mount is hardened to lower resiliency by supplying a pressure fluid to a bellows only when the tractor is moved backward without the trailer to automatically tilt the coupler main body.

A bellows 12 expanded and contracted by the supply and discharge of a pressure fluid is provided between the lower surface of the front side of a coupler main body 5 and the upper surface side of a chassis frame 4 of a tractor 1, and a pressure fluid supply pipe 13 and a pressure fluid discharge pipe 14 are connected to the lower end of the bellows 12 to supply the pressure fluid from the pressure fluid supply pipe 13 to the bellows 12 or discharge the pressure fluid in the bellows 12 through the pressure fluid discharge pipe 14, whereby only when the tractor 1 is moved back without a trailer 2, the coupler main body 5 is automatically tilted backward. Thus, even if a rubber mount 11 is hardened with the aged deterioration to lower the resiliency, the tractor 1 and the trailer 2 can be connected to each other.