A breaker is provided with: a movable piece (4) that has an elastically deformable elastic portion (43) and a movable contact on the tip of the elastic portion (43) and that presses the movable contact into contact with a fixed contact; a thermal actuator element (5) that is formed into a shape with a convex surface and is deformed by a change in temperature, thereby actuating the movable piece (4) so that the movable contact separates from the fixed contact; and a resin case (7) which houses the movable piece (4) and the thermal actuator element (5). The bottom surface of the base end (42a) of the movable piece (4) is positioned lower than the top (5a) of the upper surface of the thermal actuator element (5) by a distance (D). Thereby, a stable temperature adjustability and a resistance value are ensured in a breaker used as a protection device for a secondary battery with miniturization brought into realty.