PURPOSE: To prevent leakage of exhaust gas by a method wherein a communicating hole for a main combustion chamber having a glow plug and an auxiliary chamber of heat insulating structure is opened by means of an on-off valve made of ceramic during high load running, and leakage to the outside is prevented from occurring by means of a sealing means.

CONSTITUTION: In a control device 10, during partial load running, a communicating hole 14 between a main combustion chamber 8 and an auxiliary chamber 5 of heat insulating structure is closed by means of an on-off valve 6 made of ceramic compression ratio is increased, and in a state where a glow plug 12 is energized, alcohol fuel is injected through a first nozzle 15, and gasification is promoted to improve ignitability. Meanwhile, during high load running, the on-off valve 6 is depressed to open the communicating hole 14, and a valve 30 of a valve stem 19 is brought into contact with a valve guide 21 to perform sealing. With this state, during a compression stroke, intake air produces a squish flow and flows in the auxiliary chamber 5 to produce excellent fuel-air mixture of the intake air and alcohol fuel injected through a second nozzle 17, and flame formed due to natural ignition is discharged in the main combustion chamber. This constitution prevents leakage of combustion gas to the outside.