A gas circuit breaker is configured by comprising a tank filled with arc extinguishing gas, a pair of fixed side arc contact and moving side arc contact in the tank, a puffer chamber formed with a puffer cylinder including the moving side arc contact at its end and a fixed piston, and an insulating nozzle, attached to the end of the puffer cylinder, surrounding the moving side arc contact, that forms a flow channel to guide arc extinguishing gas from the puffer chamber to the contacts. A puffer cylinder 104 has a cylindrical member 104a at a breaker side end, an end of the insulating nozzle 101 is provided in the cylindrical member 104a. And the gas circuit breaker is configured by further comprising a hollow moving side main contact 102 at a breaker side end of the cylindrical member 104a, a pressing metal fitting 103 to engage an insulating nozzle 101 in the hollow part of the moving side main contact 102.