PURPOSE: To effectively utilize various services by using an ISDN main line or specific B channel in an extension interface exclusively for a packet switching service by utilizing a packet multiplexing function.

CONSTITUTION: An ISDN extension interface part 5 detects the call setting of an ISDN packet terminal 4 and sends the call setting to a central control part 8. Then the B channel which is already used is specified for the packet switching service. When this B channel is present, the possibility of the packet multiplexing is analyzed. When there is the possibility, the control part 8 sets the set B channel in use channel information in the call setting and sends it to an ISDN network 2 through an ISDN main line interface part 3. When the B channel is not present, on the other hand, or when the packet multiplexing is impossible, the control part 8 sets a free B channel in the use channel information in the call setting and sends its call setting message to the ISDN network 2 through the interface part 3.