PURPOSE: To improve gas sealing property, by a method wherein the under surface of a fall preventing ring for supporting a mouth piece and the under surface of a cylinder head are commonly processed.

CONSTITUTION: An auxiliary combustion chamber 2 is formed in a cylinder head 1 such that it is protruded outwardly from a position where the inner edge part of a cylinder gasket 6 is disposed. An opening 11 of the auxiliary combustion chamber 2 is provided with a mouth piece 5 having a nozzle hole 7b and a fall- preventing ring 9' engaged with an outer peripheral staged part 15 of the mouth piece 5. The engaging surface of the fall-preventing ring 9' is formed in the shape of a cone, and the outer peripheral staged part of the mouth piece 5 has a slope matching the engaging surface of the fall-preventing ring 9'. This enables common processing of the under surface of the fall-preventing ring 9' for supporting the mouth piece 5 and the under surface of the cylinder head 1, resulting in improvement of gas sealing property.