A four-stroke-cycle engine of a motorcycle arranged between front and rear wheels thereof has a crank case in which a crank shaft is arranged and a plurality of cylinder assemblies operatively connected to the crank shaft and includes a front-side cylinder assembly and a rear-side cylinder assembly along a longitudinal direction of a motorcycle body. Each of the cylinder assemblies is composed of a cylinder block, a cylinder head mounted to a head portion of the cylinder block and a head cover covering the cylinder head. The cylinder assemblies are arranged so as to provide V-shapes along the longitudinal direction of the motorcycle body. A mating surface formed between the head cover and the cylinder head of at least the front-side cylinder assembly of the plural cylinder assemblies has an inclination obliquely downward towards the front wheel with respect to a mating surface formed between the cylinder head and the cylinder block of the front-side cylinder assembly.