A battery pack detachably mounted in a device for supplying power to the device to operate the device includes at least one rechargeable secondary battery (1), a battery case (31) housing the rechargeable secondary battery (1), a mounting mechanism for mechanically mounting the battery case (31) in the device, an electrical connection connected electrically to the device for supplying power to the device, and a pack circuit board (74) securing the electrical connection. The battery case (31) is divided into a first casing and a second casing, the pack circuit board (74) is secured to the inside surface of the first casing, and the mounting mechanism is installed so the mounting mechanism is at least partially exposed on the surface of the first casing.