A lock key is unlocked in a state where a cellular phone is processing a call and using an external battery as the power supply. A “HIGH” signal is inputted to a connection detection circuit before the power supply from the external battery stops. A control circuit determines, based on the voltage detected by the connection detection circuit, that a battery unit in which the external battery is disposed and the main body of the cellular phone are in a detaching-state, and switches the power supply from the external battery to a built-in battery of the cellular phone via a battery change-over circuit. In other words, because the switching process of the power supply takes place in the above-mentioned detaching-state after unlocking the lock key, the power supply to the cellular phone is not interrupted, and the power-on state is kept, and the call, etc. are continued.