To provide an underfloor heat storage air-conditioning system that utilizes cold air generated in a heat-pump water heater, while exhibiting high cooling efficiency, thereby minimizing operation of an air conditioner during summer.

The underfloor heat storage air-conditioning system includes a heat storage means, a heat-pump water heater 7, cold-air introduction passages 51, 55, outside-air introduction passage, cooling air passages 20, 30 and an exhaust passage 52. During nighttime, cold air generated in the heat-pump water heater 7 is introduced to an underfloor space 4A to cool a heat storage means. During a daytime, the outside air introduced into the underfloor space 4A via the outside-air introduction passage is cooled by the heat storage means and used for cooling. The underfloor space 4A is formed of an inside space of solid foundation concrete 40 which is formed by surrounding all side surfaces and a bottom with concrete, in at least a lower half thereof. The most of the heat storage means is the foundation concrete 40. In a portion where the outside-air introduction passage and an exhaust passage 52 are arranged in parallel to each other, a heat-exchanging ventilator 6 is disposed to exchange heat of the outside-air introduced in daytime with the exhaust, to be supplied to the underfloor space 4A.