In a communication system, a first mediation apparatus includes a first control device. The first control device stores in a memory a correlation database correlating a mediation apparatus ID of the second mediation apparatus with a first device ID of a device. The first controller requests a management server to register the first device ID, and receives a first command from the management server. The first controller transmits a second command to the second mediation apparatus through the firewall by using a series of procedures. The second command includes second instruction information based on the second command. The first controller transmits a first response to the management server. The second mediation apparatus includes a second control device to initiate the series of procedures and transmits a third command based on the second command to the device, and transmits a second response as to the first mediation apparatus through the firewall.