NOVELTY - The breaker has a fixed piece i.e. base terminal (41), comprising a stationary contact. A movable piece (4B) comprises a movable contact (3) in a front-end part of an elastic part (43) to elastically deform. A thermal-actuation element operates the movable piece such that the movable contact separates the stationary contact by deforming in connection with a temperature change, where the fixed piece, the movable piece and the thermal-actuation element are accommodated. The movable piece comprises a biting-in nail (95, 96) that cuts into a case i.e. resin base.    USE - Small-sized breaker for use in a secondary battery circuit of electric equipment e.g. thin-shaped multifunctional mobile telephone such as notebook-type personal computer, tablet-type portable information terminal apparatus and smartphone, utilized as safety circuit board i.e. protection apparatus, for electric machines i.e. domestic electrical appliance (all claimed), and motor vehicle.    ADVANTAGE - The breaker allows the case to be prevented from hot air in another case by assigning the latter case side with respect to the hot air of a reflow process such that the deformation of the former case can be suppressed, so that a positional relationship of the movable contact provided at the front-end of the movable piece before, and the stationary contact currently embedding under the former case can be stabilized, thus maintaining temperature characteristic of the breaker favorably while reducing contact resistance of contacts at electricity supply time. The breaker allows the movable piece to be accommodated in the case from a back surface such that the front-end can protrude in a surface side of the movable piece, so that the biting-in amount at the front-end is large and coupling of the movable piece and the case is firmed, thus preventing detachment of the movable piece, and hence responding to heat processing in a reflow process.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a movable piece.    Movable contact (3)    Movable piece (4B)    Base terminal (41)    Elastic part (43)    Biting-in nails (95, 96)