To provide a power unit less affected by the length of the cylinder of an engine. A cam shaft drive mechanism is composed of a cam shaft drive pulley, a belt, a pair of cam shaft pulleys, and a tensioner disposed on the front surface of a power unit. Since the cylinder is disposed in the width direction of the vehicular body with its cylinder axis substantially in the horizontal direction (for example, a tilt angle .alpha. with respect to the ground is set at +10), the center of gravity of the vehicle is lowered and also the cylinder length can be set within the vehicular width. This increases the degree of design of the vehicle.