To provide a small-sized breaker having a wider contact gap between a fixed contact and a movable contact and thereby capable of suppressing occurrence of sparks between both contacts even if same external dimensions as a conventional one are maintained.

In the breaker composed of a fixed member 3 having a fixed contact 31, a movable member 2 having a movable contact 21, a thermally-actuated element 4 of a reverse type, and a positive temperature coefficient thermistor 5, a recessed surface of a curvature face of the thermally-actuated element 4 is made to cover the positive temperature coefficient thermistor 5 at a usual time of non-overheating, and the positive temperature coefficient thermistor 5 is of a sheet shape having a protruded part 52, and moreover, the protruded part 52 occupies a space demarcated inside the recessed surface of the curvature part. The movable contact 21 is further pushed up by a pushing margin r of the protruded part 52 at the time of overheating to increase a gap g1 between the contacts and sparks are suppressed.