A step board is attached to a main body in a rotatable manner. The step board is biased toward the upper side due to a tensile force of a spring while taking a position at which the step board is attached to the main body as an axis, and an attachment angle thereof is so fixed by a stopper as to fall within a predetermined angle. On the lower side of the step board, a cylinder unit covering a side surface of the bar-like main body is provided. The cylinder unit is connected to a brake shoe. Accordingly, when the step board is stepped on, the cylinder unit is moved downward so that the brake shoe makes contact with part of a rear wheel, thereby limiting a rotational velocity of the rear wheel.