To advance the pivotally supporting point of a swing arm on a vehicle body frame, the distance in the forward and backward direction between a crankshaft and a countershaft of a transmission is decreased as much as possible thereby making the length of the swing arm great and sufficiently absorb upward and downward oscillation of a rear wheel to achieve stabilization of the vehicle body.A countershaft is disposed below a main shaft of the transmission while a shift drum is disposed rearwardly of the main shaft, and a driving sprocket wheel for driving a rear wheel is provided on the countershaft. An internal combustion engine is disposed on the vehicle body frame such that a rear portion of a transmission is positioned above a pivotally supporting point of a swing arm. By the configuration, the pivotally supporting point of the swing arm can be advanced with respect to the internal combustion engine, and also the movable range of a rear portion of the swing arm is increased and the variation of the posture of the vehicle body can be reduced.