PURPOSE: To increase the output of an engine, by lowering the pressure in a crank chamber when a piston is moved downward and raising the pressure in the crank chamber at the time of scavenging stroke.

CONSTITUTION: When a piston H is moved downward, a crankshaft A is turned and a frame E is pushed by a cam B. Here, since the frame E is connected to a valve F, the entire body of the valve F is moved downward and the volume in a crank chamber is increased, so that pressure in the crank chamber is lowered. At the time of scavenging stroke, on the other hand, the frame E is pushed upward by the cam D and pressure in the crank chamber is raised, so that scavenging of combustion gas can be facilitated.