PURPOSE: To improve the efficient use, economy and space saving capability of an interface channel by executing protocol conversion between plural basic interfaces, which constitute a multi-line hunting group by the same number, and a terminal on the same device.

CONSTITUTION: When call originating requests are received from terminals T1-Tn, a processor 1 observes the use condition of B channels, which are connected from an interface part 4 to a communication network side respectively, and speedily sends a set-up message through a D channel handler, which corresponds to an idle line, to the communication network. When a call proceeding message is received from the communication network, a path is set between the B channels of the idle lines. Accordingly, even when the both B channels in the specified lines, the B channel can be used in case that the B channel of the other line is idle. Then, a circuit and function in one terminal adapter can be shared as well.