PURPOSE: To increase the degree of freefrom of designing while improve strength by enclosing a ceramic of a chamber part with a metal by means of casting, forming a cylindrical projected part on this cast metal, and shrink-fitting a ceramic of an adaptor part into said projected part.

CONSTITUTION: The sub-chamber of an internal-combustion engine, which is made of a PSZ sintered body, etc., consists of a chamber part 2 of a nearly semispherical hollow body having a fule-injection nozzle inserting port 5 and a glow-plug inserting port 6 on its top part, and a deep quadrilateral adaptor part 3 made of an Si3N4 sintered body, etc., having an injection port connected with a main combustion chamber. When a metal part 1 is to be casted enclosing the outer periphery of the chamber part 2, a cylindrical projected part 1a having an inside diameter smaller than the outside diameter of the adaptor part 3, is also molded, and the adaptor part 3 is shrink-fitted into the projected part 1a. Thereby, a sub-chamber configuration suited to the characteristic of an engine can be selected, while making the most of the characteristic of a celamic as a sub- chamber.