PURPOSE: To prevent the generation of crack of the outer wall by providing a first and a second cooling water jetting passages in a partitioning wall for separating an intake port from an exhaust port along the inner wall surface of a cylinder head so as to be opened in the outer wall of a subsidiary chamber, and enlarging a part of the cooling water jetting passage near the opening in the direction vertical to the inner wall surface of the cylinder head, as it comes closer to the outer wall of the subsidiary chamber.

CONSTITUTION: A piston 3 is inserted into a cylinder block 1, and a main combustion chamber 6 is provided between the inner wall surface 4a of a cylinder head 4 and the top surface 3a of the piston 3. A partitioning wall 27 for partitioning an intake port and an exhaust port 26 is provided in the cylinder head 4, and a first cooling water jetting passage 29 is provided at position far from the inner wall surface 4a, and a second cooling water jetting passage 29 is provided at a position near there, and openings 30, 31 are provided so as to face to the outer wall 7 of an subsidiary chamber 13. The jetting passage 29 are formed so that a part of each passage near the openings 30, 31 is formed so as to be enlarged in the direction vertical to the inner wall 4a of the cylinder head 4 as it comes closer to the subsidiary chamber outer wall 7. A space between both the jetting center lines 301, 31a is gradually enlarged to facilitate the collision of the cooling water with the subsidiary chamber outer wall 7 to improve the cooling property, and the generation of crack is prevented.