In order to control efficiently a flash memory in the case where small-scale data are frequently rewritten, a method of controlling a flash memory in which a data storage region is divided into unit sectors and data can be erased by every sector as a unit, is configured that the sector comprises a sector control region and a plurality of pages, and included are both available/unavailable flag information about the page concerned and an occupied/unoccupied map showing a data storage location of the data storage region on the page concerned. By employing such a configuration, it is easy to decide whether the page referred to is available or unavailable and the configuration is suitable for frequently renewing and adding pages, which are of a large quantity and on a small scale.