In a system where a communication device performs secure communication by using a digital certificate, to enable a device of a communication party to verify that a self certificate is certainly generated by a device indicated on the self certificate even if the self certificate is not delivered offline in advance. Based on a master key and a public parameter, a communication device generates an ID-based encryption private key for which a device unique ID is used as a public key. Then, the communication device generates the digital signature of an RSA public key as a ID-based encryption signature by using the ID-based encryption private key. Then, the communication device generates an RSA self signature for the RSA public key, an expiration date, a host name, the device unique ID, and the ID-based encryption signature as the target. Then, the communication device generates a self-signed certificate to include the ID-based encryption signature and the RSA self signature.