An encryption process is employed in the LSI design so as to improve confidentiality of the circuit design data over conventional examples. In the encryption process, confidential circuit design data is encrypted to produce encrypted design data and a cipher key. The encrypted design data is provided to the user who conducts a design/verification process. The key is also provided as required. In the design/verification process, the encrypted design data is subjected to various processes without disclosing the contents of the original circuit. In a decoding process, the encrypted design data subjected to the design/verification process is decoded to produce original circuit design data.