A multibit NAND Flash memory (1) includes a first storage unit (9) having a plurality of first blocks as data write regions; an instructing unit (13) that issues a write instruction of writing data into the first blocks; a converting unit (14) that converts an external address of input data to a memory position in the first block with reference to a conversion table in which external addresses of the data are associated with the memory positions of the data in the first blocks; and a judging unit (15) that judges whether at least one of the first blocks does not store valid data, wherein the instructing unit (13) issues the write instruction of writing the data into a first block in which the largest amount of data is written, among the first blocks storing no valid data, when such first blocks exist.