A data processing apparatus includes a data processing part, a power supply connector, and an original power supply device connected to the power supply connector and selected from among a commercial power source adapter outputting a direct-current voltage generated from a commercial power source, a battery unit, and a connection device coupled to an automobile power source. The apparatus includes a DC/DC converter which generates DC power supply voltages from an original power supply voltage supplied from the original power supply device via the power supply connector, and a power supply switching unit which supplies the original power supply voltage and the direct-current power supply voltages to the data processing part when one of the commercial power source adapter or the battery unit is connected to the power supply connector and which supplies only the direct-current power supply voltages generated by the DC/DC converter to the data processing part when the connection device coupled to the automobile power source is connected to the power supply connector.