PURPOSE: To accommodate many terminal equipments without increasing number of time slots in one transmission frame by allocating an idle time slot in the transmission frame to a terminal interface causing a data transmission request with an exchange section every time a data transmission request arrives from the terminal interface.

CONSTITUTION: An exchange section 10 allocates an idle time slot in a transmission frame to terminal interfaces 51-5n causing a data transmission request every time the data transmission request arrives from the terminal interfaces 51-5n, the terminal interfaces 51-5n allocates a transmission data to the time slot allocated itself from the exchange section 10 and transmits the result. As a result, the time slot is allocated to the terminal interfaces 51-5n only at the data transmission. Thus, it is possible to accommodate lots of terminal equipments B1-Bn without being limited to the time slot number of one transmission frame and the extension request of the terminal equipments B1-Bn is sufficiently replied.