In a communication system having a client and a server capable of communicating with each other via a network, the server can transmit a command to the client in response to a command request transmitted from the client to the server. The server may include a replying unit that transmits replaying data in response to the command received from the client in such a manner that (a) if there is a command to be transmitted to the client, replying data including the command is transmitted to the client, and (b) if there is not a command to be transmitted to the client, replying data, which does not include a command is transmitted to the client. Further, the replying unit may transmit the replying data without a command to the client when a no-command status lasts a predetermined standby period after receipt of the command request.