BREAKER, SAFETY CIRCUIT AND SECONDARY BATTERY PACK

The breaker is provided with a fixed contact a movable piece having an elastic portion and a movable contact a thermally-actuated element deforming with a change in the temperature so as to shift the movable piece from a conduction state to a cut-off state, a case main body housing the movable piece and the thermally-actuated element a lid member attached to the case main body and a plate-shaped cover piece embedded in the lid member The case main body is provided with a first fixing surface fixed to the lid member and the lid member is provided with a second fixing surface fixed to the case main body The second fixing surface is provided with a protruding portion protruding toward the first fixing surface and the protruding portion and the cover piece are overlapped with each other in the plan view as viewed in the thickness direction of the cover piece