PURPOSE: To advance a prefetching start point so as to restore when data overrun occurs by prefetching data out of a low speed device so as to store it in the buffer of a data transfer device and transmitting the data to a high speed device.

CONSTITUTION: Where each record R has a record length of 1 K byte, and the data transmission speed of a disk controller 2 to a disk drive device 3 doubles that of a channel device 1, the disk device 2 prefetches data from the channel device 1 by a time T. If a LOCATE RECORD command specifies only 0.5 K byte the half the average record length due to the error of a program, the prefetching time halves, that is, T/2. When a R2 is transmitted, data is insufficient, and the overrun occurs. In this case, when the average record length is made sufficiently large and the prefetching start point is advanced, the occurrence of the overrun is prevented, and data is securely transmitted.