Measuring devices each include a rotating body arranged between a wheel and a wheel hub with rotation centers aligned with each other, and an optical detector fixed to a body frame. The rotating body is formed of a disk portion attached between the wheel and a mounting surface of the wheel hub, and a cylindrical portion extending from the disk portion to the body frame and encompassing an outer circumferential surface of the wheel hub. In a circumferential surface of the cylindrical portion, there are provided elongate holes as light-transmitting portions that transmit light therethrough. The optical detector includes a light-emitting element serving as a light-emitting portion and a light-receiving element serving as a light-receiving portion, which are fixed to the body frame by a mounting bracket.