To prevent combustion gas injected into an intake passage from remaining therein and prevent the combustion gas from blowing through a cylinder into an exhaust passage without stopping in the cylinder during a valve overlap period.

A cylinder head 3 of a gas engine includes the intake passage 4 opened/closed by an intake valve 6 and the exhaust passage 5 opened/closed by an exhaust valve 8. In order to supply the combustion gas into the cylinder 1 after injecting the combustion gas into the intake passage 4 and mixing the same with intake air, a gas injection nozzle 11 for injecting the combustion gas toward the intake passage 4 is provided in the vicinity of an intake seat ring arranged on a cylinder opening portion 4a of the intake passage 4.