PURPOSE: To facilitate mounting various kinds of parts on which traction forces are exerted by providing a baseplate connected to a chassis via at least two fixing means in an end area located at a tractor, and providing the baseplate with one fixing position.

CONSTITUTION: Bearing bushes 21 are provided at the right and left of the underside of a U-shaped member 11 which is located at a tractor when a baseplate 1 is attached to the tractor, and a pin 22 supporting the major axis support member of a chassis is fitted into each bush 21. A column 3 serving as a generally vertically extending fixed position is fixed in the end area of the baseplate 1 facing the opposite side to the tractor, and a pin 51 is freely inserted from sideways into a mouth-shaped part 31 formed in the column 3, to connect an adapter plate 5. An opening 52 is formed in the pin 51, and a pin 32 vertically displaceably provided in the column 3 is engaged in the opening 52. Further, teeth 34 for engagement with a lock pin 35 are formed on the peripheral surface of the pin 32 to function as restraint means.