PURPOSE: To reduce the memory capacity by storing a telephone number of a data transmitter whose data transmission is allowed to a 2nd data transmitter, releasing a telephone line to a 1st data transmitter when a received number is coincident with a stored number and calling a data transmitter having the coincident number.

CONSTITUTION: The line connection is implemented between an NCU (network controller) section 2 and an NCU section 10 by using a public line 8 and the NCU section 2 sends a telephone number of its own. The telephone number received by the NCU section 10 is recognized by a dial recognition section 11 and collated with a telephone number stored in advance in a memory 12. When the same telephone number is stored therein, the lie is once opened and the NCU section 10 makes a dialing to the received opposite telephone number to connect the line between the NCU sections 2 and 10. Then data transmitters 1, 9 are in communication enable state and operated to make data transmission between terminal equipments 21, 22 via MODEM sections 7, 15.