Abstract: A semiconductor IC 1 is provided with an address input terminal 3 to which addresses for reading secret data stored in the semiconductor IC are supplied from the outside; an input terminal 7 to which a switching control signal C1 for selecting the addresses inputted to the address input terminal 3 is supplied; an arithmetic circuit 6 for performing an arithmetic operation according to a predetermined secret rule on secret data D1˜Dm which are read out; and a test output terminal 8 for outputting results E1˜Ek of the arithmetic circuit 6 to the outside of the semiconductor IC 1. Therefore, the secret data D1˜Dm can easily be read to the outside when the semiconductor IC 1 is subjected to a test or the like, while maintaining confidentiality of the secret data.