A gas circuit breaker includes a trigger electrode which is arranged to be movable between a first arc contactor and a second arc contactor, a compression chamber for pressurizing arc-extinguishing gas, the compression chamber being formed by a cylinder which has an outer wall and an inner wall, each being formed in a cylindrical shape, and which is provided to the second arc contactor, and a piston that slides between the outer wall and the inner wall in conjunction with the trigger electrode, and an insulation nozzle that guides the arc-extinguishing gas pressurized in the compression chamber to an arc ignited between the first arc contactor and the second arc contactor. The insulation nozzle is supported by the inner wall of the cylinder.