PURPOSE: To attain the use of channels efficiently by monitoring real data amount from each data terminal equipment at the terminal adaptor.

CONSTITUTION: After a control section 4 makes connection of a channel corresponding to a communication speed of a data terminal equipment, a data monitor section 2 monitors a real data amount from a DTE I/F section 1 and the control section 4 makes connection/interruption of an information channel B with call control through a D channel based on the real data amount information. Furthermore, in the case of transmission of burst data in excess of the operation limit of each channel, it is devised that no data loss is caused by way of the use of a buffer for tentative data storage provided to the data monitor section 2 and by way of the flow control by the DTE I/F section 1.