PURPOSE: To enhance reliability by connecting a hydraulic cylinder and an opening/closing arm to one end of a first lever with a connecting pin, connecting one end of a second lever to a tube of hydraulic cylinder with a pin, and connecting the other end thereof to the other end of the first lever.

CONSTITUTION: An opening/closing arm 4 is installed to a pin 5 in such a manner that it can be freely opened and closed. Next, a piston rod 6a is connected to the upper part of opening/closing arm 4 with a pin 7, and the bottom of hydraulic cylinder 6 is connected to the upper part of another opening/closing arm 4 with a pin 8. When an offset load B acts on the tip of opening/closing arm 4, this force produces force C which moves the hydraulic cylinder 6 to the left at the pin 8. The horizontal component of this force is applied to a guide pin 14, but the leftward movement of hydraulic cylinder 6 is hindered. As a result, the hydraulic cylinder 6 lowers in a condition that it is kept horizontal in relation to a body 2.