When a client certificate is required to connect to a client according to a predetermined communication protocol, a server receives a connection request that is a message associated with specific information from the client in a handshake for connection with the client. The server makes a client judgement as to whether or not a cache hit occurs, which is to find a summary client certificate in a cache area using the specific information associated with the connection request. When the result of the client judgement is true, the server returns to the client a connection response including hit information indicating that a cache hit occurs.