An air-cooled engine is hung and fixed under a rear part of a backbone of a body frame. The engine is disposed so that the center axis of a cylinder in its cylinder block extends approximately horizontally and so that its cylinder head is directed toward the front. A fuel injection device is located between an intake valve and an intake port in the cylinder head. An injection nozzle of the fuel injection device and an intake valve opening are placed at a short distance. While the vehicle is idling or running at low speed, the vicinity of the tip of an injector of the fuel injection device is cooled by assist air supplied from a throttle body through a secondary passage, and when the vehicle runs normally or at high speed, the fuel injection device is cooled by the wind that hits the cylinder head.