PURPOSE: To supply a communication method with which the efficiency of data communication and the secrecy of data can be improved in a situation where the functions of mutual systems are known.

CONSTITUTION: At the time of executing full-duplex communication between a call MODEM and incoming MODEM whose mutual communication systems are known, the mutual negotiation sequences for mutual function recognition of call MODEM and incoming MODEM are saved before communication starts, and data communication 12 is executed. Thus, the MODEM of access cannot be executed from the MODEM of a general protocol which executes data communication 12 after the required time of the mutual negotiation sequences is shortened and the mutual negotiation sequences are executed.