PURPOSE: To always apply emergency communication by comparing the priority of a 2nd communication MODEM with that of a 3rd communication line MO DEM to attain the communication with one communication MODEM which is higher in priority than the other.

CONSTITUTION: When a 1st communication MODEM 1 and a 2nd communication MODEM 2 are in communication, a total control section 21 of a 3rd communication MODEM 3 controls a network control section 19 to call the 1st communication MODEM 1, then the total control section 13 sends a line switching signal to an exchange network 4, thereby connecting public lines 5, 7. Then the total control section 21 sends a priority signal to the public line 7 and a priority signal detection section 12 detects the priority signal from the 3rd communication MODEM 3 and compares the priority with that of the 2nd communication MODEM 2. When the priority of the MODEM 3 is higher than that of the MODEM 2, the total control section 13 keeps the public line 5, 7 connected. Thus, the communication with high urgency is ensured.