A power unit includes a shaft member rotatably supported by the crankcase and a torsional coil spring, disposed at one end of the shaft member, biasing a swing arm in one rotational direction. An oil strainer is fixed to the crankcase through a strainer support bracket extending vertically relative to a strainer element. The torsional coil spring includes a coil portion wound around the shaft member, one end portion retained on the side of the shaft member, and the other end portion retained by a retaining portion formed integrally with the strainer support bracket.