An improved combustion chamber of a subchamber type of internal combustion engine defines a space extending contiguously to a subchamber injection port within a subchamber mouthpiece. A subchamber injection port control rod is disposed in the space adjacent the injection port. Remote from the subchamber injection port is an actuator within the space. A working substance such as wax, rubber, alcohol, etc. that expands and contracts to a large degree when subjected to temperature changes is sealingly enclosed within the actuator by an expansible member made of rubber or the like. One end of the subchamber injection port control rod is inserted into the expansible member. During a light loading operating state of the engine, a tip end portion of the subchamber injection port control rod is positioned to lie substantially along the same plane as the passageway wall surface defining the subchamber injection port, while during a heavy loading operating state the control rod is projected into the subchamber injection port and reduces the cross-sectional area of the passageway.