A gas circuit breaker includes a first arc contactor electrically connected to a first lead-out conductor, a cylindrical guide portion provided on a second lead-out conductor side, a trigger electrode arranged to be movable between the first arc contactor and the guide portion, and igniting an arc along with a movement in a first half of a current breaking action, a compression chamber being formed by a cylinder having an outer wall and an inner wall both formed in a cylindrical shape, and a piston sliding between the outer wall and the inner wall, and an insulation nozzle guiding the arc-extinguishing gas to an arc ignited at the first arc contactor. The insulation nozzle is formed integrally with the inner wall of the cylinder.