A pre-code device includes firstly memory circuit, an address decoder, and an alternative logic circuit. The first memory circuit includes a number of memory blocks and at east a replacing block. The memory blocks are pointed by a number of respective physical addresses. The replacing block is pointed by a replacing address. The address decoder decodes an input address to provide a pre-code address. The alternative logic circuit looks up an address mapping table, which maps defect physical address among the physical addresses to the replacing address, to map the pre-code address to the replacing address when the pre-code address corresponds to the defect physical address. The alternative logic circuit correspondingly pre-codes the pre-code data to the replacing block.