A screw mounting hole and a female screw part are formed in a bearing part of a gear cover, and the screw insertion hole is formed in the idle shaft. The idle shaft is fixed to the gear cover by inserting a fixing screw through the screw mounting hole and screwing it into the female screw part through the screw insertion hole. A positioning groove in a predetermined position in relation to the screw insertion hole is formed in the end face of the idle shaft. When the positioning groove is disposed perpendicularly to an engine mounting surface and the idle shaft is fitted to the gear cover, the screw mounting hole is opposed to the screw insertion hole and, when the fixing screw is fitted, the confirmation of hole positions and the aligning of holes can be eliminated.