There is provided a communication system in which a public key certificate is easily updated when identification information is changed, while network traffic increase is prevented. A print server executes encryption communication using a public key certificate which attests that own public key corresponds to own identification information. Then, if the print server detects that the own identification information (IP address or host name) is to be changed, the own server certificate which attests that own public key corresponds to changed identification information is generated by an electronic signature using the CA private key in a server.