To improve lubricity and seizure resistance of a piston.

A bolt inserting hole 33 for penetrating through these cylinder and cylinder head, is formed in the cylinder 14 and the cylinder head 21. The cylinder 14 and the cylinder head 21 are fixed to a crankcase 12 by a fastening bolt 35 inserted into this bolt inserting hole 33. Clearance is formed between the fastening bolt 35 and the bolt inserting hole 33, and lubricating oil from an oil pump is supplied to a camshaft 25 via this clearance. An oil feeding hole 61 penetrating toward a cylinder bore 16 from the bolt inserting hole 33 is formed in the cylinder 14, and the lubricating oil is directly supplied to an outer peripheral surface of the piston 17 stored in the cylinder bore 16 from this oil feeding hole 61.