The objective of the present invention is to provide a 2-cycle gas engine and a fule gas injection system used by the engine. The 2-cycle gas engine is provided with a cylinder; a cylinder head; a piston that is accommodated in the cylinder and demarcates a combustion chamber between the cylinder head and the peripheral wall of the cylinder; a fuel gas injection means that injects fuel gas to the combustion chamber; a scavenging port that is arranged on the peripheral wall of the cylinder and supplies air to the combustion chamber when the piston is positioned in the vicinity of bottom dead center; and a fuel gas injection timing control means that is configured in a manner so as to cause the injection of fuel gas by means of the fuel gas injection means when the piston is in an up stroke and the piston is positioned 0-100 degrees before top dead center, and furthermore to cause the injection of fuel gas by means of the fuel gas injection means when the piston is positioned in the vicinity of top dead center. The fuel gas injection means comprises a first fuel gas injection means which injects fuel gas to the combustion chamber when the piston is positioned in the vicinity of top dead center, and a second fuel gas injection means which is separated from the first fuel gas injection means and injects fuel gas to the combustion chamber when the piston is positioned 10-100 degrees before top dead center.