PURPOSE: To prevent the occurrence of unnecessary write-back to a main storage device by adding information, which indicates whether write-back to the main storage device is necessary or unnecessary, to information stored in a buffer storage device.

CONSTITUTION: A directory part 21 consists of a page number 12, a valid bit 13, and abandonment bits 14 (d15 to d0), and abandonment bits 14 are the means where information indicating whether abandonment is possible or not is recorded. When information in the buffer storage device is accessed, it is decided by a control means whether abandonment is possible or not, and the result is recorded in the recording means 14. Consequently, when it is necessary to expel information from the buffer storage device, the record indicating whether information to be expelled can be abandoned or not is checked by suppressing means 15, 16, and 17; and when the record is true, write-back is suppressed. Thus, unnecessary write-back to the main storage device is prevented.