PURPOSE: To prevent a pressure wave due to the generation of an arc at the time of current cutting off from spreading to a mechanism portion side so as to enhance cut off capacity by providing an insulating barrier which interlocks with a movable contact so as to isolate the mechanism portion from an arc-extinguishing device.

CONSTITUTION: When an overcurrent flows through a circuit and a tripper 17 works so as to release the catch receiver 14b of a mechanism portion 14 and a movable contact 13 opens and separates from a fixed contact 12, an insulating barrier 22 is lifted upward integrally with the movable contact 13 and both edge portions side in a groove portion 19 and the lower end edge portion gets out of a groove portion 20 and the lower end side portion is brought into sliding-contact with a projecting strip 21 so as to overlap each other so that the mechanism portion 14 side in a basket body is cut off and isolated from an arc-extinguishing chamber 15. Accordingly, the sudden rise of pressure in the arc-extinguishing chamber 15 due to the generation of an arc does not spread to the mechanism portion 14 side, and the arc is maintained in a stretched state by the magnetic drive of de-ionizing grid 16 so that the cut off time is reduced so as to be able to enhance cut off capacity.