An RSA decryption apparatus that is used in an IC card or the like counters a differential fault attack. The RSA decryption apparatus computes at high speed a public key used in data verification, without having to obtain the public key from an external source. The RSA decryption apparatus includes a remainder computation unit 412 that calculates dp=d mod (p−1), and an inverse computation unit 414 that finds an inverse of dp over a residue field with p−1 as a modulus. The RSA decryption apparatus verifies a decipher text with use of the inverse of dp as the public key. The reduced bit count in inverse computation compared to if the inverse of d is found as the public key increases computing speed.