A puffer type gas circuit breaker comprises a first and a second arcing electrodes movable relative to each other, an insulating nozzle surrounding the second arcing electrode, a puffer device for introducing an arc-suppressing gas into the insulating nozzle to puff out the arc produced between the two arcing electrodes when the breaker is in motion for circuit breaking, and a movable shield device for relieving the concentration of an electric field at the front end of the first arcing electrode during the breaker motion for circuit breaking. When the breaker is closed and the circuit made, the movable shield device surrounds the first arcing electrode and insulating nozzle, with a predetermined gap between itself and the insulating nozzle opposite thereto, and is kept at the same potential as the first arcing electrode. The circuit breaker further includes a device for maintaining the predetermined gap and the same potential at least in the early stage of the breaker motion for current breaking.