PURPOSE: To release a residual path surely, by writing ∞ information to an address of a pad control memory corresponding to a time slot in vacant display with a time slot vacancy display table.

CONSTITUTION: Subscribers SA∼SD are connected via time division multiplex channel TDSW consisting of time switches TSWF0 and TSWF1, space switches SSW0 and SSW1, digital pads DPAD0 and DPAD1, and time switches TSWB0 and TSWB1. A controller (not shown) reads out a time slot vacant display table in a main memory at each prescribed time, writes ∞ information to an address in pad control memories PSM0 and PSM1 corresponding to the time slot displaying vacancy, and completely release the channel path of the said time slot. Thus, the residual path which is caused from the discidence between the content of the said display table and the content of the pad control memory due to any cause, can be eliminated.