A relay unit includes: a storing unit storing a first correspondence relation between interface information of a client and a tenant identifier of the client, and a second correspondence relation among the tenant identifier, connection information about a connection with a server, and a state of connection with the server; a tenant identifying unit obtaining the tenant identifier from the first correspondence relation; a connection identifying unit creating the first connection information and adding the correspondence relation among the created first connection information, the obtained tenant identifier, and the state to a busy state into the second correspondence relation; and a connection creating unit creating a connection with the server and transmitting the obtained tenant identifier and the message to the server, wherein the connection identifying unit sets the state to an unused state in response to a reply message from the server.