Breaker, safety circuit, and secondary battery pack

A breaker comprising:

• a fixed piece having a fixed contact;
• a movable piece having a plate-shaped and elastically deformable elastic portion and a movable contact which is formed at a tip portion of the elastic portion and is pushed to be in contact with the fixed contact;
• a thermally actuated element which deforms in accordance with a temperature change to thereby shift the state of the movable piece from a conductive state in which the movable contact is in contact with the fixed contact to a cut-off state in which the movable contact is separated from the fixed contact;
• and
• a case which accommodates the fixed piece, the movable piece, and the thermally actuated element, wherein
• the movable piece is cantilevered and supported by the case on a side of a base end portion of the elastic portion,
• the elastic portion includes a first narrow portion having a width dimension in a short direction perpendicular to a longitudinal direction of the elastic portion,
• the width dimension of the first narrow portion is smaller than a width dimension in the short direction of the base end portion,
• the first narrow portion has an arc-shaped contour in a plan view seen from a thickness direction of the elastic portion,
• the width dimension of the first narrow portion is gradually and continuously decreased with respect to the base end portion,
• the elastic portion is provided with a first protrusion protruding toward the thermally actuated element on the side of the base end portion to be in contact with the thermally actuated element in the cut-off state, and a second protrusion protruding toward the thermally actuated element to be in contact with the thermally actuated element in the cut-off state and arranged closer to the tip portion than the first protrusion,
• an end edge of the first narrow portion is arranged between the first protrusion and the second protrusion,
• the end edge of the first narrow portion is arranged on the side of the base end portion of the elastic portion, and
• a minimum width portion, at which the width dimension of the first narrow portion is the smallest, and a width dimension of the minimum width portion is 80% or more and 90% or less of the width dimension of the base end portion.