PURPOSE: To quickly increase a spring load against a large shock and to improve the riding comfortableness in an irregular road travel by winding a torsion coil spring on the rocking shaft of a swing arm, and regulating the change of the diameter of this spring with a cylindrical guide member.

CONSTITUTION: A torsion coil spring 17 wound on a rocking shaft is torsionally deformed against the rocking of a swing arm 1 while the coil diameter is changed. When the swing arm 1 is rocked by a fixed quantity or above, a guide member 20 is brought into contact with the torsion coil spring 17 to regulate the diameter change of this portion. The torsion coil spring 17 deforms only the portion separated from the guide member 20 thereafter, thus the increasing rate of a spring load is increased. A large shock can be coped with by quickly increasing the spring load, and the riding comfortableness in an irregular road travel can be improved.