There is provided a gas circuit breaker that can appropriately ensure a pressure and a density of arc-extinguishing gas to be sprayed to an arc, and can more surely maintain electric insulation performance. A gas circuit breaker 1 includes a first arc contactor 21 electrically connected to a first lead-out conductor 7a, a cylindrical guide portion 41 provided on a second lead-out conductor 7b side, a trigger electrode 31 which is arranged to be movable between the first arc contactor 21 and the guide portion 41, and which ignites an arc generated between the first arc contactor 21 and the trigger electrode along with a movement in a first half of a current breaking action, a compression chamber 36 for pressurizing arc-extinguishing gas, the compression chamber being formed by a cylinder 42 which has an outer wall 51 and an inner wall 52 both formed in a cylindrical shape, and which is provided on the guide portion 41 side, and a piston 33 that slides between the outer wall 51 and the inner wall 52 in conjunction with the trigger electrode 31, and an insulation nozzle 23 which guides the arc-extinguishing gas pressurized in the compression chamber 36 to an arc ignited at the first arc contactor 21. The insulation nozzle 23 is formed integrally with the inner wall 52 of the cylinder 42.