PURPOSE: To attain data transmission reception without limit of communication opposite party by receiving a communication data from a data terminal equipment with an interconnection circuit signal, sending the data to a protocol conversion means, applying protocol conversion to the data and sending the result to the opposite terminal equipment via a digital network.

CONSTITUTION: A data terminal equipment 10 sends a connection request with a digital interface converter 40 applying protocol conversion suitable for an opposite communication terminal equipment at first to an exchange 3 and sends a communication data X to a communication processor 22 via an interconnection circuit 12. Upon the receipt of the signal, the interface device 20 processes an interconnection circuit signal Y at the control processor 24 or the communication data X by the communication processor 22 respectively and sends it to a digital transmission reception circuit 26 in the equipment. Moreover, when a transmission reception circuit 32 receives the communication data X and the interconnection circuit signal Y, the signal is sent to the digital interface converter 40 via a time division switch 36.