A data read request for a logical address Nlog is supplied from a host system to a memory apparatus. A data processing portion calculates a physical block number Nphy corresponding to the logical address Nlog using a logical-physical conversion criterion value NBASE and a logical-physical conversion multiplier NMUL of the mapping reference information. By referencing an unusable block correlation table, it is determined whether the physical block number Nphy represents an unusable block. When the physical block number Nphy represents an unusable block, a substitute block number replaces the physical block number Nphy. The data located at the physical block number Nphy or at the substitute block number is then read from a memory portion. The read data is supplied from the data processing portion to a communicating portion which supplies the read data to the host system.