A spark ignition gas engine provided with a spark plug, characterized in that the gas engine comprises a fuel inlet connecting piece connected to a mounting hardware piece provided in a cylinder head for housing the spark plug, thereby supplying fuel gas from the fuel inlet connecting piece to a pre-combustion chamber through a fuel passage provided in the mounting hardware piece, the fuel gas supplied in the pre-combustion chamber being ignited by spark discharge at the spark plug that is fitted on a fitting seat surface in the mounting hardware piece; wherein the gas engine further comprises: a solenoid valve that performs open-close control of the fuel gas which streams into a fuel inlet passage bored in the fuel inlet connecting piece; and a first check valve that is provided posterior to a discharge side of the solenoid valve in the fuel inlet passage, so that the combustion gas generated in the pre-combustion chamber is prevented from flowing backwards in a direction from the pre-combustion chamber toward the solenoid valve; characterised by a second check valve arranged at a location just above the pre-combustion chamber in the fuel passage in the mounting hardware piece, thereby preventing the combustion gas generated in the pre-combustion chamber from flowing backwards, from the pre-combustion chamber toward the fuel inlet passage; and the first check valve arranged on the fuel inlet passage in the fuel inlet connecting piece, wherein the second check valve and the spark plug are arranged in the mounting hardware piece, so that a center axis of the second check valve is parallel to a center axis of the spark plug; a plurality of cooling grooves or bore-cooling holes is provided in the mounting hardware piece; and a tip part of the fuel inlet connecting piece is inserted in a circumferential wall surface of the mounting hardware piece, so that the cooling grooves or bore-cooling holes face an outlet end of the fuel inlet connecting piece. a solenoid valve that performs open-close control of the fuel gas which streams into a fuel inlet passage bored in the fuel inlet connecting piece; and a first check valve that is provided posterior to a discharge side of the solenoid valve in the fuel inlet passage, so that the combustion gas generated in the pre-combustion chamber is prevented from flowing backwards in a direction from the pre-combustion chamber toward the solenoid valve; characterised by a second check valve arranged at a location just above the pre-combustion chamber in the fuel passage in the mounting hardware piece, thereby preventing the combustion gas generated in the pre-combustion chamber from flowing backwards, from the pre-combustion chamber toward the fuel inlet passage; and the first check valve arranged on the fuel inlet passage in the fuel inlet connecting piece, wherein the second check valve and the spark plug are arranged in the mounting hardware piece, so that a center axis of the second check valve is parallel to a center axis of the spark plug; a plurality of cooling grooves or bore-cooling holes is provided in the mounting hardware piece; and a tip part of the fuel inlet connecting piece is inserted in a circumferential wall surface of the mounting hardware piece, so that the cooling grooves or bore-cooling holes face an outlet end of the fuel inlet connecting piece.