PURPOSE: To permit the detection of the flow rate of hot-water without any malfunction by a method wherein a temperature difference of flowing hot-water is detected by hot-water temperature sensors installed at the fore side and the rear side of the flow direction of hot-water of a thermal insulating heater in a circulating passage whereby the flow rate of the flowing hot-water is detected.

CONSTITUTION: A first hot-water temperature sensor 29 is a thermistor or the like for detecting the temperature of hot-water and is installed at the before side of a thermal insulating heater 13 in the flow direction or hot-water and is connected to a control device 31 through an A/D converter 30. A second hot-water temperature sensor 34 is installed at the rear side of the thermal insulating heater 13 in the flow direction of hot-water and is connected to the control device 31 through another A/D converter 35. When the shortage of water is generated during the operation of a cleaning device, the temperature difference of the hot-water, which is detected by the first hot-water temperature sensor 29 and a hot-water temperature detected by the second hot-water temperature sensor 35, becomes big while the value is compared with a preset temperature difference data in a memory device ROM whereby the shortage of water can be detected.