A relay apparatus for relaying communication between a water heater main body and a radio remote control unit, comprising: a switch section for setting an address of the radio remote control unit; a control section for establishing communication with the water heater main body and commanding and controlling to read out maintenance information stored in the water heater main body assuming that input information at the switch section is a request from the radio remote control unit having an address which was set by operation of the switch section; and a display section for displaying the maintenance information received from the water heater main body.