PURPOSE: To enable to connect a front and a rear frame in a compact manner, by a method wherein front and rear frames divided from each other are connected to each other at the upper and lower positions in the manner of a hinge, each of connecting pins in supported rotatably and swingably, and the upper connecting pin can be moved in a horizontal direction.

CONSTITUTION: When a steering operation is performed by a steering cylinder 7, the front frame 4 is swiveled with the connecting pins 1, 9 as a center to perform steering. In this case, since the pins 1, 9 are borne by spherical bearings 3, 10 and a reactional force of a cylinder 7 directly acts on the rear frame 6, the swiveling or steering is performed smoothly. When the road is slanted or rugged, the frames 4, 6 are swung. In this case, when the front frame 4 is swung, the pin 1 remains in the upright position, a fitting arm 2 is oscillated around the bearing 3, and an upper fitting arm 13 is oscillated while performing a circular arc motion. When the oscillation proceeds to the maximum limit and the upper edge 12a of a slanted guide slot 12 collides against a coolar 11, the collar 11 is rotated and brought into contact with a wall surface 12b, thereby damping the the shock at the time of collision.