To shorten a processing time and to reduce a re-transmission processing time when a fault occurs to min. by continuously transmitting plural tracks, recognizing the responses when data is backed-up to a sub-center from the side of the main center with a line and reproducing only data of the track whose response is not recognized when the fault occurs.

FCU at the side of the main center 1 corresponds to the data updating requests of the plural tracks from HOST, divides the plural tracks into packets and continuously transmits them to the sub-center 2 with the line. FCU at the side of the sub-center 2 repeats receiving the transmitted packets, assembling them into the original tracks and transmitting the responses to the main center 1 whenever assembling is completed. In this case, the track whose response is not received even after a prescribed time elapses among the plural tracks which are transmitted at the side of the main center 1 is detected, the detected track is divided into the packets and they are continuously transmitted to the sub-center 2 again with the line.