A battery pack includes a cartridge insertion section to which a cartridge having a function expansion device for expanding the functions of an electronic device can be inserted; a function expansion terminal for connection with the cartridge inserted into the cartridge insertion section; and an expansion function operation device for operating the function expansion device of the cartridge connected to the function expansion terminal in accordance with a control signal of the electronic device. A function expansion cartridge includes connection terminals for connection with predetermined terminals provided in the slot of the battery pack; a communication device for performing data communication with the electronic device via the connection terminals; a function expansion device having hardware and/or software for expanding the functions of the electronic device; and an expansion function control device for controlling the function expansion device in accordance with a control signal from the electronic device.