To enable a traction pin to be locked without giving the force to pull out the traction pin from the connection position caused by oscillations generated when a vehicle runs or pushing-up from a tracted vehicle, if occurs, to the side of hydraulic cylinder where oil leakage is liable to occur thereby.

Under the condition where a traction pin 7 is locked to a coupling bracket 19 of a vehicle rear side and at the connection position with a tracted vehicle, a rod 17 of a hydraulic cylinder 9 is further moved forward to perform the over-stroke, and a connection pin 33 to connect the rod 17 to a link mechanism 11 is moved in an oblong hole 21c fromed on the side of the mechanism 11. To the pin 33 one end of a wire accomodated in a wire tube 5 so that it can move is connected while the other end thereof is connected to a lock member 39 formed on the side of the bracket 19. According to the movement of the pin 3 in the hole 21c the lock member 39 is pushed through the wire 37 and turned to lock the pin 7.