A technique is provided to dynamically change priority, according to content of communication data, when the communication data is relayed between a client device and a host device. A communication gateway (102) is a relay system for relaying the communication data between a thin client device (101) and the host device (103). The communication gateway (102) monitors communication performed by the host device (103) in response to a request from the thin client device (101). The communication gateway (120) determines priority of the communication that is returned to the client device by the host device (103) in response to the request, based on a protocol for the communication performed by the host device (103).