There is disclosed a controller included in a flash memory system attachable to a memory interface of a host system. If a block in which new data is written and a block in which old data is written are present simultaneously, the controller determines in which block the newest data is written, based on version identifier stored in a redundant area of a flash memory. Specifically, the controller has an access control function for controlling access to the flash memory, an address managing function for managing correspondence between a logical block address supplied from the host system and a physical block address in the flash memory, a version identifier setting function for writing version identifier in the redundant area of the flash memory, and a version identifying function for identifying a block in which newest data corresponding to a same logical block address is written based on the version identifier.