Provided is a breaker capable of further downsizing without impairing rigidity and strength of a case. The breaker 1 comprises a fixed contact 21, a movable piece 4 having and pressing a movable contact 41 to the fixed contact 21, a thermally responsive element 5 for moving the movable piece 4 to separate the movable contact 41 from the fixed contact 21 by deformation thereof responding to temperature change, a case for containing the fixed piece 21, the movable piece 4 and the thermally responsive element 5, and a cover piece 8 attached on the case 7. The case 7 has an end face 72 on which the cover piece 8 is disposed, a containing recess 73 caved from the end face 72 and forming a space to which the movable piece 4 and the thermally responsive element 5 are contained, and a first protrusion protruding from the end face 72 and to which the cover piece 8 is fitted.