PURPOSE: To improve service to terminals by adding a priority control part to plural terminal controlling devices and integrating priority assigned to each transmission message and inherent priority in each terminal to decide its priority.

CONSTITUTION: A terminal #i and message priority Mpi are fetched from a circuit controlling part 36 and inputted to a priority controlling part 38 and its priority is recorded. After discriminating whether a data buffer 31 is idle or not, the data buffer 31 is occupied in case of idle status. Subsequently a terminal having the highest priority is read out from the controlling part 38 and signal transmission to the read terminal device is executed by a transmission means controlling part 33.