PROBLEM TO BE SOLVED: To provide a water heating device surely preventing the occurrence of clogging of a drain pipe resulted from the generation of slime or the like due to bacteria increase when arranging the drain pipe in bathtub piping in a double pipe method.SOLUTION: Tap water from an inflow path 33 is led into an introduction path 61 by opening change of an opening and closing selector valve 62, silver ions are mixed with the tap water in a silver ion mix tank 63 to form washing water, and the washing water is let flow in the drain pipe 17, which is inserted in a return pipe 30a in the double pipe method, through a drain connection pipe 16 by a drain pump 14. A period for letting the washing water flow, a flow rate or a melting amount of silver ions is varied and set according to the level of an outside air temperature by an outside air temperature sensor 65. The inside of the drain pipe 17 may be purged by taking in air from an atmosphere communication pipe 523 by continuous operation of the drain pump 14 after the washing treatment.
