PURPOSE: To allow the adjustment of exhaust gas by forming, in a space between two adjacent recesses, a passage opening one recess and communicating with an exhaust channel and accepting the stem of an exhaust valve in the passage.

CONSTITUTION: When a piston 1 is rotated inside a housing formed by two shell halves 24a, 24b, recesses 5a-5d in the piston are matched to uplifts 26a-26d on the housing, respectively, in sequence, both of which form a combustion chamber 34 in cooperation to cause combustion with spark plugs 30 provided on the uplifts, a fuel injector 31 and an air injector 32. The receiving faces of the recesses are larger at the front ends 7 than at the rear ends 6 and so thrust with combustion is directed to the front end faces 7 so that a rotor can be rotated in F-direction. When an exhaust valve having a valve head 12 and a valve stem 13 provided in a passage 58 formed between two adjacent recesses is open, exhaust gas is discharged through the rear end faces 6 and exhaust channels 15 to a manifold 35.