PURPOSE: To effectively use a time slot by allocating a digital tone generator and a time slot for conference trunk at every generation of a call in a conference room without fixing them in the allocating means of the digital tone generator and the time slot for conference trunk of a digital exchange.

CONSTITUTION: By performing hooking by a terminal 1 during the call between the terminal 1 and a terminal 3, the event information of the hooking is detected by a call control means 11, and it is informed to a time slot (TS) managing means 21. The TS managing means 21 allocates a DTGn2 to deliver a holding tone to the time slot TS1 of the terminal 1 in a TS managing table, and writes and records a code n2, and also, arranges the information of a TCn2 at the time position of the TS1 by controlling a TS allocation part 20. In such a way, a second dial tone is sent to the terminal 1, and the holding tone is sent to the terminal 3. Thus, by allocating the DTG to the TS being used by the terminal, it is possible to realize the connection of the DTG, and also, it is possible to perform the connection of a trunk for conference in a conference call.