A data processing device such as an IC card is switchable between a terminal mode in which drive electric power and signals are supplied via exposed connection terminals and an RF mode in which drive electric power and signals are supplied via built-in antennas. The data processing device has an internal circuit for being supplied with drive electric power and signals from either the connection terminals or the antennas depending on the mode to which the internal circuit has been set. The internal circuit typically comprises a microprocessor. The data processing device also has a mode detector for setting the internal circuit to the RF mode in response to a predetermined radio wave applied to the antennas and setting the internal circuit to the terminal mode in response to a reset signal applied to a reset terminal which is one of the connection terminals.