To prevent knocking by converting natural gas to easily ignitable reformed fuel, supplying the same to an auxiliary chamber and igniting the same, and supplying hardly ignitable natural gas to a main chamber.

This gas engine is provided with a heater 19 and a fuel reforming catalyst 18 in a fuel reforming device 9, and ignition fuel is supplied to the auxiliary chamber 2 through an ignition fuel supply port 16 by opening an ignition fuel supply valve 3 to improve ignitability of natural gas. Then, a natural gas supply valve 7 arranged in a natural gas supply port 21 and a water injection valve 6 are provided in an intake passage 14, and lean air fuel mixture formed of air and natural gas supplied by opening the natural gas supply valve 7 is supplied to the main chamber 1 to prevent knocking.

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