PURPOSE: To prevent uneven wear between piston and cylinder, by forming an arcuate space between cylinder and exhaust passage so that heat of exhaust gas is not transferred to cylinder.

CONSTITUTION: In an engine having a plurality of cylinders arranged in lines, an arcuate space 12 is formed by the side of lower cylinder 2 and exhaust passage 8 of upper cylinder 1 is extended at the back of said arcuate space 12. The space 12 can be disposed either near the outer surface of cylinder 2 or in the wall surface a little spaced from cylinder 2. Further, the space 12 can be communicated to the atmosphere. With such an arrangement, it is enabled to prevent local overheating and abnormal deformation of cylinder 2, so that wear of cylinder and piston can be minimized. Therefore, it is prevented that engine efficiency is lowered or the service life is shortened due to leakage of compressed gas.