PURPOSE: To evade a busy state of circuits, to shorten a waiting time and to effectively utilize the number of communication channels by providing an empty channel information table and making a data communication between network connecting devices.

CONSTITUTION: A CPU 201 controls the whole connecting device, and a ROM 202 is stored with a coded program which controls a program and ISDN controller 203 and a LAN controller 204. A RAM 205 is stored with the empty channel information table. Then the empty channel information table is updated each time a call is connected or disconnected, and the current empty state is reported to a connection request source; and a call-out side acquires the empty state of the communication channel of a call-in side in advance and makes a call to the empty channel again on the basis of the information to make a call connection for the original data communication. Therefore, when there is an empty communication channel among the limited communication channels, the call connection can be securely made.