PURPOSE: To suppress useless call and to exclude the useless use of a backup line by replying to an incoming call in response to the presence of a usual line on the restart of the usual line whose usage is once stopped.

CONSTITUTION: Computer communication is started by applying power in the inter-computer communication between host computers interrupting the inter- computer communication while the power is interrupted once. In the host computer system starting up early, when the communication is implemented via a usual line 24 and when a fault detection section detects that the communication is disable from the line 24, and an automatic dial function section makes dialing to an opposite host computer via an exchange network 26. The computer receives an incoming call at its control section 60 via a transmission/reception section 66 and selects the communication line from the line 24 to the backup line by means of a changeover section 34. Simultaneously the control section 60 commands the referencing of a display flag 54 of a control section 42 to a reference section 64 and sends the result of reference to the control section 62 and when display is registered, the communication is entered.