A gas circuit breaker having a main fixed element and a main movable element capable of contacting and moving apart from each other, a fixed contact provided on the main fixed element side, a movable contact provided on the main movable element side and capable of contacting and moving apart form the fixed contact, an insulation nozzle provided on the main movable element side to surround the contacts, a hollow nozzle provided inside the movable contact, a cylinder, a puffer cylinder and a fixed piston provided on the main movable element. The insulating nozzle serves to compress a gas in a puffer chamber defined by the puffer cylinder and a fixed piston and to guide the gas to the gap between the contacts disconnected. The circuit breaker also has an opening formed in a side surface of the cylinder to discharge the gas from the insulation nozzle, a gas flow passage formed to provide a passage between the hollow nozzle and the opening, and an exhaust guide provided outside the main movable element to close the opening. The exhaust guide is divided into a plurality of guide members connected by releasable fixing device. One of these guide members can be moved relative to the others when the fixing device is released.