To realize a processor that uses an RS232C communication port to communicate data to a connected opposite-side processor through modems and a public telephone line and to an opposite-side processor connected directly through a cross cable without using a switching means such as a switch.

After being powered ON, the processor starts connection-party prescribed processing 200 first, turns ON a data terminal ready signal line ER and a request-to-send signal line RS (steps 11 and 12), and then confirms that an opposite device is connected and operable from ON states of a data set ready signal line DR and a clear-to-send signal CS (steps 13 and 14). Then the processor judges if the opposite device is a directly connected processor or a modem from the state whether a carrier detect signal CD is ON (steps 15A to 15C). Then when the signal line CD is OFF and the opposite device is a modem, modem connection processing 300A is carried out thereafter and when the signal line CD is ON and the opposite device is a directly connected processor, direct connection time processing 400A is performed thereafter.