To prevent abnormal ignition and fire propagation inhibition from occurring by avoiding the formation of ridge lines in the circumference of an ignition plug and providing a connecting face connecting respective conical faces to each other in a cylinder head.

A ceiling wall face of a combustion chamber is formed of a closed end of a recess 21. A part of the recess 21 is constituted of a pair of air intake side conical faces 22 continuing to the both intake valves 10, a pair of exhaust side conical faces 23 continuing to the both exhaust valves 13, and flat squish area forming faces 24, 25. In the circumference of the ignition plug 18 arranged in an approximately central part of the combustion chamber, first- fifth connecting faces 28, 29, 30, 31, 32 are formed in the ceiling wall face of the combustion chamber, namely, the recess 21 in such a state as cutting ridge lines formed by crossing respective conical faces 22, 22, 23, 23 with each other. An acute angle part is thus eliminated from formed in the ceiling wall face of the combustion chamber. This constitution enables the prevention of abnormal ignition by a heat spot or of the fire propagation inhibition from the ignition plug.