To provide a bypass switch capable of activating a closed circuit operation promptly even at the time of battery cell over discharge.

A bypass switch is equipped with a first and a second fixed electrodes as a pair of electrodes connected in parallel to cells of the battery, a shaft which can be displaced in a perpendicular direction to the fixed electrodes, a movable electrode which short-circuits the cell through direct electrical connection to the fixed electrodes by being pressed in a perpendicular direction when the shaft is displaced, and a thermally actuating device, energized by the shaft, and provided with a heat-generating semiconductor element sandwiched by heat insulating members, and solder. The heat actuating device is connected to the fixed electrodes.