A gas blast circuit breaker comprises an insulation nozzle (8) for blowing extinguishing gas to an arc (9) generated between a stationary contact (1) and a movable contact (2). The nozzle (8) has a throat section (10) into and out of which one of the two contacts (1, 2) is movable and a divergent section (13) provided downstream of the throat section (10). A slanting surface (11) for increasing a reflectivity of energy intensity of the arc (9) is formed on the divergent section (13) of the nozzle (8). The nozzle (8) is formed of a fluoroplastic material and boron nitride powder not more than 15 vol.% added as a filler.