A gas engine in which active gas flows into an interior of a cylinder through scavenging ports, and fuel gas that has been mixed with the active gas is combusted, the gas engine comprising: fuel injection units each of that includes fuel injection ports located on an outer side of the cylinder, and that inject fuel gas through the fuel injection ports into the active gas drawn into the cylinder through the scavenging ports; and an opening degree adjustment unit that mechanically or electrically changes an opening degree of the fuel injection ports in accordance with a value of a parameter that varies together with a scavenging pressure (Psc), or in accordance with the scavenging pressure (Psc), characterized in that a fuel gas main pipe communicates with a fuel tank, and also communicates with a toroidal pipe via a fuel injection device, when a valve of the fuel injection device is opened, the fuel gas inside the fuel gas main pipe flows into the toroidal pipe, the toroidal pipe is disposed on an upper side of the scavenging ports on an outer side in a radial direction of the cylinder, and extends toroidally in a circumferential direction of the cylinder so as to encircle the cylinder, the fuel injection units are fixed to the scavenging port side of the toroidal pipe, the fuel injection units are provided with: an inner pipe in which inner holes are formed, the inner holes penetrating from an internal portion of the inner pipe to an external portion thereof, and the inner pipe in which fuel gas is guided to the internal portion through the toroidal pipe; and an outer pipe in which outer holes are formed, the outer holes penetrating from an internal portion of the outer pipe to an external portion thereof, and the outer pipe internally housing the inner pipe and forming a double pipe together with the inner pipe, the fuel injection ports are formed by the inner holes and the outer holes, and the opening degree can be varied using a mutual overlap of the inner holes and the outer holes. fuel injection units each of that includes fuel injection ports located on an outer side of the cylinder, and that inject fuel gas through the fuel injection ports into the active gas drawn into the cylinder through the scavenging ports; and an opening degree adjustment unit that mechanically or electrically changes an opening degree of the fuel injection ports in accordance with a value of a parameter that varies together with a scavenging pressure (Psc), or in accordance with the scavenging pressure (Psc), characterized in that a fuel gas main pipe communicates with a fuel tank, and also communicates with a toroidal pipe via a fuel injection device, when a valve of the fuel injection device is opened, the fuel gas inside the fuel gas main pipe flows into the toroidal pipe, the toroidal pipe is disposed on an upper side of the scavenging ports on an outer side in a radial direction of the cylinder, and extends toroidally in a circumferential direction of the cylinder so as to encircle the cylinder, the fuel injection units are fixed to the scavenging port side of the toroidal pipe, the fuel injection units are provided with: an inner pipe in which inner holes are formed, the inner holes penetrating from an internal portion of the inner pipe to an external portion thereof, and the inner pipe in which fuel gas is guided to the internal portion through the toroidal pipe; and an outer pipe in which outer holes are formed, the outer holes penetrating from an internal portion of the outer pipe to an external portion thereof, and the outer pipe internally housing the inner pipe and forming a double pipe together with the inner pipe, the fuel injection ports are formed by the inner holes and the outer holes, and the opening degree can be varied using a mutual overlap of the inner holes and the outer holes. an inner pipe in which inner holes are formed, the inner holes penetrating from an internal portion of the inner pipe to an external portion thereof, and the inner pipe in which fuel gas is guided to the internal portion through the toroidal pipe; and an outer pipe in which outer holes are formed, the outer holes penetrating from an internal portion of the outer pipe to an external portion thereof, and the outer pipe internally housing the inner pipe and forming a double pipe together with the inner pipe, the fuel injection ports are formed by the inner holes and the outer holes, and the opening degree can be varied using a mutual overlap of the inner holes and the outer holes.