Confidential CRC codewords are stored along with confidential data words on a ROM. A checker subjects one of the confidential data words, which has been read out from the ROM, to the same type of calculation as a predetermined calculation that was carried out to produce associated one of the confidential CRC codewords. A comparator compares a result of the calculation performed by the checker to the associated confidential CRC codeword that has been read out from the ROM.