A device for applying a check bit to encrypt instructions for protection includes a check-bit generator, a first check-bit location generator and a check-bit insertion unit. The check-bit generator generates a check bit in accordance with an instruction with a plurality of bits to be outputted. The first check-bit location generator generates an insertion position N (positive integer) for the check bit in accordance with the instruction and a predetermined algorithm. The check-bit insertion unit inserts the check bit in a position between (N-1)th-and Nth-bit of the instruction in accordance with the insertion position N generated by the first check bit location generator, thereby generating an encrypted instruction.