PURPOSE: To secure highly reliable data by preventing the transfer of erroneous data and old data even when no effective flag of a track buffer is cleared due to unexpected accidents, etc.

CONSTITUTION: This buffer control device is provided with a track buffer 80 for storing data read out of a disk 200 equivalent to plural tracks, a buffer control means 30 for controlling the writing and reading of the track buffer in accordance with an instruction from a host device and a buffer management means 20 for managing the condition of the track buffer. It is also provided with an erasing means for invalidating the managing information of the data stored in the buffer management means 20 and rewriting the data itself in the track buffer into a preset erasing pattern when a command to directly access to the area of the data of the disk or a buffer erasing command is sent from the host device relating to the data stored in the track buffer 80.