PURPOSE: To increase the arc voltage and to simplify the structure by providing an insulator to generate a pyrolytic gas by the heat of the arc at the portion exposed to the arc generated at the contact member.

CONSTITUTION: A contact member 2 and an insulator 4 are furnished in an arc extinguish chamber, and the contact member 2 is composed of a fixed contact 6 with a fixed contact point 5 and a movable contact 8 with a movable contact point 7. When an abnormal current is generated, for example, the contact 8 is opened by a trip device of a circuit breaker, and an arc is generated between the contact point 7 and the contact point 5. When the arc is generated at the contact member 2, the temperature of the insulator 4 is increased by the heat of the arc 3, and the insulator 4 generates a pyrolytic gas. The arc 3 can be cooled by the pyrolytic gas, and the arc voltage can be increased and the structure of the device can be simplified accordingly.