In response to a validation request that includes second information identifying the certificate authority, key information of the certificate authority at issuance of the public key certificate, and information identifying the public key certificate, if the second information identifying the certificate authority included in the validation request corresponds to the first information identifying the certificate authority included in the authority certificate, and the information identifying the public key certificate included in the validation request does not exist in the revocation information, the validation server creates a validation result indicating that the public key certificate corresponding to the information identifying the public key certificate included in the validation request is valid.