PURPOSE: To prevent the connection of an AC adaptor from being forgotten by generating a warning sound or a warning message in the case the AC adaptor is not connected to a personal computer in a state that an extension unit is connected, based on information of a status line for showing a fact that the power is supplied from the AC adaptor.

CONSTITUTION: A power unit 5 outputs a status for showing a fact that the power is supplied from an AC adaptor through a status line 6. When a power source switch is turned on, a CPU 7 decides whether an extension unit 2 is connected to a main body 1 or not through an input/output port 16. In such a case, when it is confirmed that the extension unit 2 is connected, the status from the AC adaptor 3 is read through an input/output port 8. In such a case, unless the AC adaptor 3 is connected, a warning message is written in a display controller 11, displayed on a display device 12, and simultaneously, a warning sound is generated by a loudspeaker 14. In such a way, it can be prevented to forget the connection of the AC adaptor, and such an inconvenience as a data loss, etc., is eliminated.