In the key installation system for improving the confidentiality and concealment of the key, a first decrypting circuit decrypts an encrypted key EKEY1(EDK(MK1)) using an encrypted key EDK1(MK1) as the key. A second decrypting circuit decrypts an encrypted key EMK1(KEY1) using the output of the second decrypting circuit, that is, an internal key KEY1 as the key. A third decrypting circuit decrypts the encrypted key EDK1(MK1) using the output of the second decrypting circuit, that is, an internal key MK1 as the key, to generate a final secret key DK1.