PURPOSE:To facilitate maintenance by supplying electric power to a radio transmitter when the temperature of bathtub water is high based on the signal from a temperature sensor which is provided for the bathtub water. CONSTITUTION:A power control circuit 20 supplies the power from a battery 21 to a transmitter 17 when temperature is higher than a specified value based on the signal from a temperature sensor 18. The preset temperature value is set at the value which is slightly lower than the ordinary temperature of the fed hot water in a bathtub 1. Namely, the hot-water feeding into the bathtub 1 is started from a hot-water feeding machine 4 at the preset hot water temperature. Until the hot water level reaches the temperature sensor 18, the temperature sensor 18 detects atmospheric temperature which is lower than the ordinary hot water temperature. Therefore, a radiowave is not transmitted. When the hot water level rises up and the temperature sensor 18 is immersed into the hot water, the output of the power control circuit 20 becomes ON. The radiowave which is modulated with a water level signal is transmitted. The radiowave is received with a receiver 22. The frequency is detected, and the water level can be found. The hot-water feeding into the bathtub 1 is controlled in correspondence with a preset water level.
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