Abstract: The present invention helps to develop a smaller sized portable terminal without reducing the functions available for the user. A USB device which is detachable from the portable terminal is provided with a USB connector which is inserted into a USB part of the portable terminal, a power source terminal which comes into contact with a power supply terminal of the portable terminal, when the USB connector is inserted into the USB port of the portable terminal, and a screw which is engaged with a screw hole of the portable terminal. When the screw is engaged with the screw hole of the portable terminal, a switch inside the screw hole is pushed down due to a contact with the screw. The portable terminal detects mounting of the USB device by an output from the switch, and initiates power supplying from the power supply terminal of the portable terminal to the power source terminal of the USB device.