Abstract: A thin U-shaped rear axle guide plate made of steel is detachably attached along rear end portions of rear fork arms made of light alloy and formed in a bifurcate shape. This rear axle guide plate is made longer at its lower branch portion than its upper branch portion, and a free end portion of its lower branch portion includes a projection projecting upward. The rear fork arms are provided at side surfaces opposite to the rear axle guide plate with a retaining pin projecting therefrom. Engaged with this retaining pin is a cam face of a drive chain tension adjusting plate pivotably supported by the rear axle. The adjusting plate is pressed and held by a nut which is in threaded engagement with the retaining pin. The adjusting plate may be formed with an annular rib around an opening in which the rear axle penetrates, and this annular rib may be engaged with step portions formed at the rear fork arms.