To provide a reexchanging energy-saving building system reducing the bills of electricity, water, hot water, and air-conditioning and using pressurized fire hydrant water and rain water.

A reexchanging energy-saving building system includes: an electricity supply system 2 of a building 1, the system performing supplying water from a roof-top pond 21 to a capillary generation system 23 for generating electricity, supplying the electric energy generated by the capillary generation system 23 to a building-use power load 27, supplying the electric energy generated by a solar generation apparatus 24 to a water-supply pump 26 for pumping up water from an underground pond 22 to the roof-top pond 21, and supplying electric energy utilizing off-peak power 25 of utility power to the water-supply pump 26; a hot-water supply system of the building 1 supplying the building 1 with hot water at any time via a parallel connection to reservoir tanks of a solar water-heater 31; an air-conditioning system for the building performing cooling and heating using an underground-heat heat pump; and a tap water-saving system processing water from residual bath water, laundry, face washing, vegetable washing using an isolation film filtration tank to supply a flush toilet downstairs with the processed water as flushing water.