PURPOSE: To improve the utilizing efficiency of a channel idle/busy memory and to facilitate the management of a line memory with respect to the idle/busy management system on the channel idle/busy memory managing the channel idle/busy state of each channel in an exchange accommodating plural subscriber lines whose channel capacity differs.

CONSTITUTION: The above exchange is provided with a line memory generating means 200 monitoring a channel idle/busy memory 100 periodically, retrieving a channel memory not secured for a subscriber line accommodated in an exchange and generating a line memory corresponding to a channel capacity as large as possible, and a line memory allocation means selecting and securing an unsecured line memory of a capacity corresponding to a channel capacity of a new subscriber line in the line memory generated by the line area generating means and releasing the secured line memory for a removed subscriber line when the accommodated subscriber line is removed.