PURPOSE: To improve efficiency of an engine and a level of exhaust gas by changing the 2-cycle engine partially into a 4-cycle operation.

CONSTITUTION: In a stage of low load and low rotational speed in an engine, an exhaust valve 2 is opened by an electromagnetic driving gear 11 in the time from before the bottom dead center to after the top dead center. Next, in a stage of lowering down a piston 8, the exhaust valve 2 is closed by the electromagnetic driving gear 11 to generate a negative pressure in a cylinder 1. In this way, air is sucked into the cylinder 1 from an intake port 3 to displace inside the cylinder 1 with fresh air scavenging inside the cylinder 1. This air is compressed and exploded by lifting the piston 8 to perform 4-stroke operation. By continuing this operation, when a pressure of exhaust gas is decreased lower than an air pressure boosted by a compressor 17, the exhaust valve 2 is controlled by a controller 20 to operate the engine as an ordinary 2-stroke engine.