PURPOSE: To secure simultaneously advantages of both an asynchronization system and a synchronization system by raising a MODEM through communication between the MODEM and a terminal equipment by the asynchronization system and implementing the communication with the synchronization system after the line with an opposite party is established.

CONSTITUTION: When communication data are sent from a terminal equipment to a public line through a MODEM in which the asynchronization/ synchronization is selected freely, at first a communication port of the MODEM is open by the asynchronization system, the MODEM is arisen by the synchronization system through the use of an asynchronization command and a signal is sent to the MODEM. After a reply message from the MODEM is confirmed, a terminal equipment closes the communication port of the MODEM. When the MODEM is arisen and the communication port is open, the terminal equipment makes communication with an opposite party through the public line by the synchronization system with the MODEM to sent/receive data. When the communication is finished, the communication port is closed to terminate the communication operation.