A rotation assistance mechanism equipped with: a variable magnetic unit having a pair of magnetic members for which the opposite poles of the members are arranged facing one another; a magnetic drive unit that changes the distance between the pair of magnetic members with a predetermined period by the rotation of a rear wheel; a shield unit having a pair of magnetic surfaces the poles of which are the same as the pair of magnetic members; a shield drive unit that advances the shield unit to between the pair of magnetic members in predetermined cycles when the distance between the pair of magnetic members is contracted, and retracts the shield unit from between the pair of magnetic members in predetermined cycles when the distance between the pair of magnetic members is expanded.