PURPOSE: To provide an external storage controller which can speed up operation by reducing the overhead accompanying a prefetch.

CONSTITUTION: The external storage controller which has a CPU 6 and an internal buffer memory 7 and accesses an external storage device 16, is provided with a transfer informing means 18 which detects the completion of a part or the whole of serial data transfer between the external storage device 16 and internal buffer memory 16 and reports that to the CPU 6. The transfer informing means 18 sends a report to the CPU 6 which controls the external storage controller, for example, when read data are stored in previously set buffer memory addresses or each time read data as many as the previously set number of bytes are stored in buffer memory addresses.