To smoothly start an engine without stopping an engine, to promote combustion in a load operation, to improve combustibility and to improve thermal efficiency.

By properly using an ignition plug 60 and a precombustion chamber 31 with pilot combustion injection valve 32 provided on a cylinder head 4, ignition within a main combustion chamber 1 is performed by an ignition plug 60 at the time of a staring, ignition by the pilot fuel injection valve 32 is performed when ignition within the main combustion chamber 1 exceeds a prescribed number of times to smoothly start a gas engine and to stabilize a continuous operation.