To aim at the reduction of weight of a device without lowering strength thereof by increasing thickness of a part, which needs strength, so as to improve the strength of a part of an upper half part of a vertical wall part in consideration of transmission of the force that a large force is applied to an upper side.

A spring bracket 2 is fitted to a side rail 1 of a frame of a truck, and one end of a rubber spring 3 is fitted to each inward inclined surface of the bracket 2. At the time of analyzing the transmission route of the force to be applied to a main member 4, the force is transmitted from tires 12, 13 to a saddle 9 through an axle 14, and the stress is concentrated to a part connected to the saddle 9. Namely, thickness of a lower part 5a of a part connected to the saddle 9 is formed larger than the thickness of other parts. With this structure, since the thickness of a part, to which the stress is concentrated, is formed large, a sufficient strength is maintained, and furthermore, since the thickness of other parts is reduced, weight of the whole of a device is reduced.