Abstract: An X.25 protocol apparatus can communicate with another X.25 apparatus or a X.32 apparatus through a telephone network, an ISDN network, or a PBX network, by attaching protocol conversion system to the X.25 apparatus. The conversion system has a pair of signal identification portions (1, 6) for separating a receive packet to a data packet and a control packet, a call process portion (3, 12) for dial signal process, and an address table (4) and a packet edition portion (5) for address conversion of a control packet. A data packet is forwarded from one signal identification portion to another signal identification portion through direct path (53). A control packet is forwarded to a control packet identification portion (2) for control process which includes dial signal process, address conversion, and protocol sequences. This system is applicable to couple a data communication network which has a X.32 correspondence portion to another data communication network.