To precisely find the types of external power supplies by comparing the fully charged supply voltage of power supply pack of detected voltage and the value of supply voltage of AC adapters to a judging reference voltage selected.

A power supply type judging circuit 1A has a voltage dividing resistors 13A and 13B, and the divided voltage at the connection midpoint P1 is connected as a detection voltage VD to a detection voltage input terminal 11C of a power supply type judging unit 11. The power supply type judging unit 11 converts a detection voltage VD from analog to digital, forms a detection voltage data, and judges whether the value is larger than the judging reference voltage data. Value of the judging reference voltage data is selected to a value higher than the output voltage of fully packed power supply pack 3 but lower than the output voltage of AC adopter 4. That is, it is judged as power supply pack when the detected voltage is lower than the judging reference voltage and identified as AC adapter when the detected voltage is higher than the judging reference voltage.