To operate a piston and rotate an output shaft by inserting a pin portion of a swing member, connecting an outer peripheral connecting portion thereof with the piston via a joint, into an oblique hole on a shaft end of the output shaft disposed on the same side as a cylinder head.

On a swing member 3 that is swingable in all directions via a spherical bearing 4, a protruded pin portion 32 is provided on an output shaft 1 side. The pin portion 32 is inserted into an oblique hole 113. In an engine, gas compressed by a piston 8 is combusted and expanded, of which the pressure in turn moves the piston 8 to an opposing combustion chamber 90. The pressing force of the piston 8 acts as the force to push an outer peripheral connecting portion 31 of the swing member 3 via a joint 7. On the other hand, the pin portion 32 with a spherical bearing 4 as the supporting point can move only in rotating direction of the output shaft 1. That is to say, the swing member 3 converts the pressing force of the piston 8 to the force to rotate the output shaft 1. Therefore, the piston 8 pushed by combusting pressure in one combustion chamber 90 conducts a compression stroke or exhaust stroke in the other combustion chamber 90.