When a free physical block where data is to be written is searched for, a search process for searching for a pair of free physical blocks is first executed using a free physical block search table. Detection of a free non-pair good block is executed only when a pair of free physical block is not detected in the search process using the free physical block search table. When there is a free physical block, two-plane write is executed. When there is no pair of free physical blocks, data is written in an adequately combined non-pair good blocks.