To retain a proper movement range for a movable iron core and prevent an erroneous operation due to vibrations and impacts by installing an auxiliary iron core which is so installed as to be brought into contact with either a fixed iron core or a movable iron core, as not to interfere the turning locus of the movable iron core, and as to be on the opposite to the other iron core.

An auxiliary iron core 31 is made of a magnetic material and installed in the outside of a fixed iron core 23 so as to be brought into contact with a pair of side plates of the fixed iron core 23. At the time when low electric current flows in an initial stage during which a movable iron core 28 is attracted by the fixed iron core 23 in the case a set value or higher electric current flows in a heater 21, the gap between the auxiliary iron core 31 and the movable iron core 28 is set to be narrower than the gap between the fixed iron core 23 and the movable iron core 28. As a result, since the electromagnetic attracting force generated between the fixed iron core 23 and the movable iron core 28 can be increased, the minimum value of instantaneous tripping current is lowered and operational stroke of the pushing part of the movable iron core 28 is surely retained and thus erroneous operation due to vibration, etc., can be prevented.