PURPOSE: To form an economical line without exclusive possession of a specific time slot for a data signal and without provision of a private line for a data signal.

CONSTITUTION: When a transmission request of a data signal comes, a central control section 80 of a master station confirms whether or not an idle time slot is in existence in time slot sections 61-6n. If the time slot section 62 is idle, a line switch section 52 is thrown to the position of a data contact and a data line switch network 40 is controlled to connect a data line interface section 21 to the line switch section 52. Simultaneously information representing that the time slot section 62 is used for a data signal is sent to a slave station via a control time slot section 100.