A vehicle comprised of a motorcycle or a motor tricycle and including a body frame, a fuel tank, an internal combustion engine, and an apparatus for guiding a vaporized fuel generated in the fuel tank into an engine oil collected in a crankcase of the internal combustion engine, the crankcase housing an oil pump for pumping up the engine oil, the crankcase defining a suction passage for guiding the engine oil to a suction opening of the oil pump, the apparatus comprising: a vaporized fuel passageway defined in the crankcase; and a vaporized fuel conduit extending from the fuel tank to the vaporized fuel passageway, wherein the vaporized fuel conduit has a first check valve for allowing only flow of the vaporized fuel from the fuel tank into the crankcase, characterized in that the crankcase includes a left case half and a right case half mated together at their respective division surfaces (67, 84), the right case half having an L-shaped-cross-sectional wall along the division surface thereof, the left case half having a lid portion extending from the division surface into a crank chamber of the crankcase, the L-shaped-cross-sectional wall and the lid portion jointly defining the vaporized fuel passageway, the vaporized fuel passageway having a lower end located below a predetermined lower limit level of the engine oil and above an inlet of the suction passage provided at a bottom of the crankcase, the vaporized fuel conduit is connected to a branching pipe via a T-shape fitting, the branching pipe having a distal end opened to the atmosphere and also having a second check valve for allowing only flow of an outside air into the vaporized fuel conduit, and the vaporized fuel conduit, as it extends from the fuel tank to the T-shape fitting, runs transversely of the vehicle substantially across a width of the body frame of the vehicle. a vaporized fuel passageway defined in the crankcase; and a vaporized fuel conduit extending from the fuel tank to the vaporized fuel passageway, wherein the vaporized fuel conduit has a first check valve for allowing only flow of the vaporized fuel from the fuel tank into the crankcase, characterized in that the crankcase includes a left case half and a right case half mated together at their respective division surfaces (67, 84), the right case half having an L-shaped-cross-sectional wall along the division surface thereof, the left case half having a lid portion extending from the division surface into a crank chamber of the crankcase, the L-shaped-cross-sectional wall and the lid portion jointly defining the vaporized fuel passageway, the vaporized fuel passageway having a lower end located below a predetermined lower limit level of the engine oil and above an inlet of the suction passage provided at a bottom of the crankcase, the vaporized fuel conduit is connected to a branching pipe via a T-shape fitting, the branching pipe having a distal end opened to the atmosphere and also having a second check valve for allowing only flow of an outside air into the vaporized fuel conduit, and the vaporized fuel conduit, as it extends from the fuel tank to the T-shape fitting, runs transversely of the vehicle substantially across a width of the body frame of the vehicle.