PURPOSE: To ensure a display of an attendant board with high efficiency, by providing a common control part of attendant board which is common to each attendant board, then connecting said control part to each attendant board via a channel.

CONSTITUTION: The extension state change information and the trunk state change information which are detected by processors 1 corresponding to attendant boards 3 respectively are sent to a common control part 6 for attendant board and then stored in a memory 9. The information stored in the memory 9 is analyzed and decided an attndant board control processor 8 and sent to a channel interface 7. The interface 7 applies protocol conversion to the data given from the processor 8 and transmits it to a channel 4. This data of the channel 4 is stored in a memory 13 via a channel interface 12 of an attendant board interface 10. The contents of the memory 13 are read out in response to the operation of the board 3.