A memory system includes a non-volatile semiconductor memory that includes a plurality of blocks, each of the blocks being a data erasing unit; an erasing time monitoring unit that monitors time required for erasing data from the non-volatile semiconductor memory; a management table for managing the erasing time on a unit of each of the blocks; and a wear-leveling control unit that levels number of rewriting across the blocks based on the management table. The memory system accommodates variations among lots, individual pieces, and blocks, thereby performing highly-accurate wear leveling.