PURPOSE: To favorably keep the mixture of a fuel and air in a combustion chamber space sandwiched between a piston and a cylinder head extending all over the combustion chamber space.

CONSTITUTION: An auxiliary chamber space 29 communicated through a communication hole 31 with a combustion chamber space 25 sandwiched between a piston 15 and a cylinder head 21 is provided in the cylinder head 21. The auxiliary chamber space 29 is divided into a vapor generation chamber 33 close to the communication hole 31 and an energy chamber 37 remote therefrom, and a stepped portion 35 is provided in the energy chamber 37. After fuel is supplied to the stepped portion 35 of the energy chamber 37 remote from the communication hole 31, fuel is supplied to the vapor generation chamber 33, so that fuel vapor in the vapor generation chamber 33 is put into a jet by pressure increase due to combustion in the energy chamber 37 and passed through the communication hole 31 to be pushed out into the combustion chamber space 25. Thus, the fuel vapor is spread in a wide range in the combustion chamber space 25 to be actively mixed with air in the whole area in the combustion chamber space 25.