To provide a low-speed 2-cycle gas engine that reduces NOx emission and prevents knocking without supplying high-pressure gas into a cylinder.

The low-speed 2-cycle gas engine includes a piston (5), a piston rod (6), a crosshead (9), a connecting rod (10), an exhaust valve (4), a scavenging port (3), and a gas injection valve (20) provided in a cylinder liner for injecting gas fuel to the center part in the cylinder liner. The gas fuel is injected to the center part in the cylinder liner from the gas injection valve, and pre-mixed with scavenging air introduced from the scavenging port for combustion operation. The gas injection valve is preferably configured to start injecting the gas fuel after the piston starts rising from a bottom dead center and immediately before the exhaust valve is fully closed.