A radio communication system of a water heater comprising a slave set which remotely operates a water heater main body by radio communication, and a master set which continuously communicates with the water heater main body by wired communication and which intermittently communicates with the slave set by radio communication to relay communication between the water heater main body and the slave set, wherein after the slave set instructs to change a hot water supply temperature of the water heater main body to a requested temperature by the salve set, the requested temperature previously instructed by the slave set is maintained as the hot water supply temperature displayed on the display section of the slave set for a particular time until the hot water supply temperature of the water heater main body reaches the temperature requested by the slave set.