PURPOSE: To previously control a breakage portion of a bimetallic strip to an ideal position by providing a weak point (stress concentration part) on a part or a peripheral part of radially arranged slits.

CONSTITUTION: In a bimetallic strip 2, a shaft supporting hole 2a for inserting and shaft-supporting an adjusting screw is provided on the center part and plural slits 2b-2g are provided radially from the shaft supporting hole 2a. Also a stress concentration part is provided in which a bottom surface 2c' of an optional slit 2c, coincident with neither a position on the center line X-axis connecting a pair of movable contacts 3 nor a position on the center line Y-axis orthogonal to the X-axis, is formed with a corner of R' radius smaller than a corner of R radius of the bottom surfaces 2b'-2g' of the other slits 2b-2g. When an overcurrent flows into a load connected in series, the metal 2 is heated with a heater 6, and when the metal 2 reaches a given temperature, it is reversed so that the movable contact 3 is quickly separated from a fixed contact 4 to break a circuit.