A fuel tank generally comprises a tank body, a fuel inlet formed to an upper portion of the tank body, and a cap provided for the fuel inlet. The tank body is provided with a recessed portion recessed in a direction separating from at least one of a vehicle body frame and a vehicle body cover covering the frame so as to form a space therebetween as an air-flow passage. The air-flow passage is composed of, from a vehicle front side, an air introducing section, an air-flow separation section and a separated air-flow section which faces an opening of the intake system of the engine.