PURPOSE: To prevent the generation of multiplex misconnection of a virtual line by sending virtual lineconnecting information simultaneously with a virtual lineconnection request information at the time of connecting the virtual line between computers.

CONSTITUTION: A virtual line 4 is connected from a computer A to a computer B through a virtual line 3. When the computer A is reset by abnormality, the line connecting information of the computer A is lost. In case of restarting the computer A and sending a virtual line connecting request 5, virtual line information 52, i.e., information indicating that the computer a is not connected to the computer B because the computer A has been once reset, to the computer B is included in the request 5 together with virtual line connection request information 51. Receiving the request 5, the computer B compares the information 52 with the self-information, detect no specification of the virtual line 4 and executes the disconnection 6 processing of the line 4. Consequently, the multiplex misconnection of the virtual line can be prevented.