mode control processing unit 102 makes a demand of the resumption of data transmission on the host communication software step 1020 and sets the changeover flag 510 corresponding to the call as the object to be changed to the off state step 1021 if a clear indication packet is received from the party step 1022 then the data communication mode control processing unit 102 release the call by which the changeover enquiry has been performed step 1023 thereby notifying a communication mode changeover complete to the host communication software step 1024 in addition the data communication based on the line switching is initiated at the time of completion of the changeover of the transmitting receiving buffers when the operation indicating code represents a changeover reject in step 1014 or a clear indication message is received form the party called in step 1017 the data communication mode control processing unit 102 resumes the transmission of a data packet 1025 and makes a demand of resumption of the data transmission on the host communication software step 1026 thereafter it sets the changeover flag 510 corresponding to the call as the object to be changed over to the off state 1027 then it is checked in step 1028 whether or not the number of retries of the changeover request reaches a predetermined value if it is judged.