PURPOSE: To increase an allowable mounting load without degrading steering performance, by pivoting front and rear frames in the direction of an axle on a pin, and providing free turning wheels on both sides of the frame at the more backward position than the position of the center of gravity of the front frame.

CONSTITUTION: The position G3 of the center of gravity of the front frame 10 is located at the more backward position than front wheels 1. At both sides of the part which is located at the more backward position than the position G3 of said center of gravity, the free turning wheels 11, which share the weight W3 of the front frame 10 together with the front wheels 1, are attached. The position G4 of the center of gravity of the rear frame 12 is located at the more advanced position than the rear wheels 2. The tip side, which is protruded to the left side from the position G4 of the center of gravity, is rotatably linked to the front frame 10 by way of the pin 13. By said pin 13, the front frame 10 and the rear frame 11 are linked so that the both can be bent only in the up and down directions.