Abstract: A combustion chamber of a sub-chamber type internal combustion engine is improved so as to operate efficiently at an excellent fuel consumption with low smoke density, either upon partial loading or upon heavy loading. The combustion chamber includes a sub-chamber port communicating a main combustion chamber with a subsidiary combustion chamber formed within a cylinder head. A space portion is formed in a cylinder head member on the side of the sub-chamber port located farthest from a cylinder center line on a cross-sectional plane of the sub-chamber port defined as a plane including a center line of the subsidiary combustion chamber and the center line of a cylinder and opening into the sub-chamber port. An expansible/contractible tube is contained within the space portion. A sub-chamber port control rod is connected to the expansible/contractible tube and slidably extends into the opening of the space portion.