Abstract: An object of the present invention is to effectively achieve the reduction in piston slap sound in both of an expansion stroke and a compression stroke in consideration of a difference between the behaviors of a piston in the expansion stroke and the compression stroke. For this purpose, in a piston P having a skirt 12 formed into an intermediately bulged barrel type, the axis L2 of a piston pin is offset with respect to the cylinder axis L1, and the decrement in radius of a lower skirt portion 20 is set to be large on a thrust side and small in an anti-thrust side. In the expansion stroke, the piston is positively swung, while the skirt 12 is put into abutment against a wall surface of a cylinder bore, thereby reducing the piston slap sound.