PURPOSE: To simplify the structure of internal-combustion engine and to reduce the weight, by providing a cylinder rotary chamber in the casing while providing an intake groove and exhaust groove in the innercircumferential wall face.

CONSTITUTION: A cylinder rotary chamber 8 is provided in the casing 1 while an intake groove 11 and exhaust groove 12 are made in the innercircumferential wall face facing with an ignition plug 10 by 180°. A piston 3 is fitted reciprocally to choke the open end of cylinder 2. While crank shaft 5 is coupled with a conrod 4 to gear the teeth of crank shaft 5 with outer teeth thus to spin and revolve. The output shaft 7 is extended integrally from one end of crank shaft 5. Consequently, the structure of internal-combustion engine is simplified while the weight is reduced.