To catch impurities in the hot water without being accompanied by deformation of a filter housing and/or to filter the hot water at a suction part.

The filter housing C has a rice ball shaped (nearly triangle shape) crosssection, even when the inside of a hot water passage 100 becomes negative pressure at an operation time of a circulation pump, and at least two pieces of ribs projecting inward and coming into contact with the outer peripheral surface of a cartridge filter A are provided in the longitudinal direction of the filter housing C, and the hot water passage 100 is formed between the filter housing C and the cartridge filter A. Thus, the deformation of the filter housing C is suppressed.