PURPOSE: To attain transmission of a 128kbps data by employing two B-channels of one basic interface simultaneously.

CONSTITUTION: A control circuit 11 sets the connection of two B channels simultaneously between ISDN terminal equipments or terminal adaptors, an identification signal control circuit 10 sends/receives an identification signal to detect a difference of transmission delay time between the two B channels before data communication after setting of simultaneous connection, stores a received identification signal to a tentative storage circuit 8 to detect the transmission delay time difference of the two B channels. A data division circuit 3 divides a 128kbps data signal from a data terminal equipment 30 received via a data terminal interface 2 into two B channels and the resulting signal is sent. A data synthesis circuit 9 decodes the two received 64kbps data signals into the original 128kbps data signal by referring the transmission delay time difference detected precedingly and sends the decoded signal to the data terminal equipment 30 via the data terminal interface 2.