PURPOSE: To avoid a repetitive call connection request by an H channel terminal equipment whose call connection is rejected because a time slot in an exchange system cannot be acquired.

CONSTITUTION: When an ISDN exchange system 1 cannot acquire time slots of a required number, a time slot number recording processing section 5 stores number of a time slot in use which cannot be acquired and a number of an idle time slot which can be acquired to a memory. A time slot release processing section 6 detects it that a busy time slot is idle. A time slot acquisition request exclusion processing section 7 excludes the idle time slot and an idle time slot acquired at first even when other call makes acquisition request. A period detection processing section 8 detects it that the busy time slots are all idle and the enable connection to relevant H channel terminal equipments 131-13n making a connection request via a primary group interface 11 is informed from a D channel processing unit 10.