To prevent adhesion (accumulation) of soot on the inner wall surface of a combustion cylinder (a combustion chamber).

Air outlets 113 are provided to blow off air to both sides in a peripheral direction along the inner peripheral surface of a second combustion cylinder 111. Thus, since blow-off directions of air blown through adjoining air outlets 113 are a peripheral direction extending along the inner peripheral surface of the second combustion cylinder 111 and since directions interfere with each other, air blown through the adjoining air outlets 113 are made to collide with each other. Further, since, in the vicinity of the inner wall surface of the second combustion cylinder 111, stagnation (a cavity area) is hardly generated, and since in the vicinity of the inner wall surface of the second combustion cylinder 111, mixing of fuel and air is promoted as the fuel is prevented from being thickened, excellent combustion is promoted, and the generation of exhaust emission, such as soot and unburnt gas, is suppressed.