PURPOSE: To improve a volume efficiency and save energy by forming an exhaust port and an exhaust valve respectively near a cone-shape and decreasing as small as possible the dead volume generated due to employment of a plate- like exhaust valve structure known in conventional cases.

CONSTITUTION: An exhaust port 11a of a valve seat 11 is formed in near cone- shaped so as to make the end face 11b thinner at a cylinder side and a valve seat connection 12a of an exhaust valve 12 is formed in near cone-shape to be fitted with the internal configuration of a valve seat exhaust port 11a having a flat bottom 12d. Accordingly, at the instance of compression finish or a piston 7 reaches the upper dead point, the flat bottom 12d of the exhaust valve 12 makes contact with cylinder side end faces 11b, 2b of the valve seat 11 or a suction valve 2 so as to be approximately aligned. Also the dead volume (space 10) formed when the piston is at its upper dead point is made almost zero. Consequently, improvement of volume efficiency and saving of energy can be attained.