A battery pack includes a battery, a pair of output terminals connected to the battery, a switch inserted between the battery and one of the pair of output terminals, an acceleration sensor detecting a shock and outputting a detected value dependent thereon, and a control unit of a protection circuit. When a detected value of the acceleration sensor is equal to or greater than a predetermined threshold value, the control unit turns off the switch and thereby cuts off a current path between the battery and the pair of output terminals to cut off the current.