A shaft of a pinion gear is engaged with a ring gear extends toward the front side in a final gear case that is provided in a power transmission system between a power unit and a wheel. The first wrench includes a shaft and two sockets each formed at an end of the shaft. An exterior cover may be releasably attached to the adapter piece and may squeeze a portion of the adapter piece as the exterior cover is releasably attached to the adapter. Having undergone processes of diffusion and attenuation, the laser light beam is then safely used to administer light therapy on a patient.