Memory system and control method thereof

A memory system comprising:

• a nonvolatile memory including a plurality of blocks as data erase units;
• a measuring unit which measures an erase time at which data in each block is erased;
• a block controller having a block table which associates a state value indicating one of a free state and a used state with the erase time for each block;
• a detector which detects blocks in which rewrite has collectively occurred within a short period;
• a first selector which selects a free block having an old erase time as a first block, on the basis of an information in the block table;
• a second selector which selects a block in use having an old erase time as a second block, on the basis of the information in the block table;
• and
• a leveling unit which moves data in the second block to the first block if the first block is included in the blocks detected by the detector.