PURPOSE: To give a suction main flow a large curvature, and to display the effect of a swirl and the effect of a squish flow geometrically by increasing the curvature of the inner wall surface of the combustion chamber guiding the main flow of a suction flow sufficiently.

CONSTITUTION: A helical type swirl port 2 is connected to the combustion chamber 1, and the main flow F of the suction flow is given a swirl. The main flow F flows into the combustion chamber 1 so as to collide with a plug section wall 5. Since the curvature of the plug section wall 5 is formed in sufficiently large concave spherical surface shape, the main flow F is turned by a large curvature, and passes through a position where sufficiently separating from a squish area 6. Accordingly, damping by the squish area 6 of the swirl of the main flow F is prevented, and combustion is improved by both the swirl and squish flows.