PURPOSE: To lessen a space occupied by an engine and reduce the wheel base of a motorcycle by forming a longitudinal center line, which passes through the cylinder center, so as to slant down forward with respect to the body frame.

CONSTITUTION: An engine E is mounted on a body frame F, and a crankshaft 1 is about orthogonal to the longitudinal direction of the frame F, while front and rear cylinder blocks 2, 3 of the crankshaft 1 are arranged, with a crank case 4 interposed between them, slantingly forward in the longitudinal direction with respect to the frame F. A longitudinal center line L-L passing through the centers of cylinders 2a, 3a of front and rear cylinder blocks 2, 3 are slanted down forward and make a tilt angle with respect to a horizontal line H-H. A transmission 13 is arranged below and in one body with the engine E, and the output shaft 14 of the transmission 13 is connected to the rear wheel W through a chain transmission mechanism 15 as usual.