An extinction cover (5) is mounted on a case (4) thereby sealing an extinction chamber (S). This extinction cover has an engaging-force-increased portion (5c1) which strengthens the force of engagement with the case, and an engaging-force-decreased portion (5c2) which weakens the force of engagement with the case. In response to an increase in internal pressure in the extinction chamber due to arc gas generation, the engaging-force-decreased portion establishes disengagement of the case and provides a gap (27) with the case, for linking the extinction chamber to external air, and the engaging-force-increased portion maintains engagement with the case.