The logic format of a mask ROM (10) is composed of a user region (11) having a plurality of blocks, a spare region (12) having a plurality of blocks, and a table region (13) having a plurality of blocks. When the outside data is written in the mask ROM by the control of a MPU (24), if there is a defective block in the user region, the defective block is replaced by a spare block in the spare region. If the spare block is defective, it is replaced by another spare block. The replacement information (address, et.) is stored in a block in the table region, and if the block of the table region is defective, it is replaced by another block. Accordingly, since a defective block in the mask ROM (10) is replaced by another block, a defective mask ROM is relieved.