PURPOSE: To drive an engine at a high compression ratio by providing a water injection nozzle in the sub chamber of an engine using gas as fuel and cooling air-fuel mixture just before self-ignition caused by compression heat by means of spray.

CONSTITUTION: A water injection nozzle 31, a spark plug 32 and a gas introducing pipe 33 are attached. to a sub chamber 3 arranged on a cylinder head 11, and spray is injected from the water injection nozzle 31 at a preset point of time in the latter half of the compression stroke, moreover compression is continued while cooling air-fuel mixture just before self ignition caused by compression heat, thereby, the engine is driven by being ignited by means of the spark plug 32 and lowering the piston 2 by pressure.