One exemplary object of the present invention is to provide a card device which can be shared in main body devices employing different power supply systems, and a power supply method of the card device. The card device according to the present invention includes a flat rectangular card base body (101), an internal circuit (301), an external connector (102), and a main body connector (103). The external connector (102) is provided on an edge (202) positioned on an opening side (201) of a card slot (21) in a state in which the card device is inserted into the card slot (21), and is adapted to be supplied with power from an external power supply. The main body connector (103) is provided on the innermost side of a card slot (200) in a state in which the card device is inserted into the card slot (200), and is supplied with power from a main body device. Power required for an operation of the internal circuit (301) is supplied from one of the main body connector (103) and the external connector (102) which is supplied with power.