PURPOSE: To provide an ISDN subscriber system which can effectively execute communication between terminals housed in an ISDN exchange by using a B channel.

CONSTITUTION: At an ISDN exchange 01, the throughput variables of channel buses B1 and B2 are monitored, the channel with no throughput variable is discriminated by this monitor, and an interruption message is sent to an NT 02. The NT 02, which receives this interruption message, interrupts the designated channel, prepares an interruption confirmation message and sends it to the ISDN exchange as a response message. The ISDN exchange 01, which receives this interruption confirmation message, turns to the waiting state of a call setting request and when the call setting request is received, a channel assignment instruction message corresponding to the request is sent to the NT 02. The NT 02, which receives this message, assignes the channel in the interruption state to the requesting terminal.