Engine unit of motorcycle

An engine unit of a motorcycle generally comprises a cylinder head, a cylinder block formed with a cylinder bore therein, a crankcase and a rear case sequentially assembled in this order from a front side of a motorcycle body. A crankshaft is pivoted to a mating face between the cylinder block and the crankcase, a counter shaft which is rotated with the crankshaft in an interlocking manner is pivoted to a mating face between the crankcase and the rear case. These two mating faces lie in planes parallel to each other so as to be substantially perpendicular to a center axis of the cylinder bore of the cylinder block.