PURPOSE: To perform effective communication against large fluctuation in calls, by avoiding the connection of calls at high speed when more than a prescribed amount of vacant lines are not available in a multiplex relay transmission line, setting lines at low speed to communicate at low speed terminals in place of high speed terminals.

CONSTITUTION: When a subscriber makes a call at a telephone terminal, a caller side terminal controller 3 transmits a telephone communication request to an exchange 4. The exchange 4 receiving it checks for the state of use of the multiplex relay transmission lines, and when the basic lines of vacant state are R+1 (where; R is a control constant) or more, the 8 basic lines are used, the high speed lines are set and the telephone call is connected. When the basic lines of vacant state are less than R, the exchange 4 uses one basic line to set the low speed line and to transmit a signal instructing to communicate at the data terminal to the controller 3, and receives an approval signal based on the approval of the caller to makes call connection of the data terminal at the low speed line.