To provide a heating system capable of constantly stably producing hot water of high set hot water supply temperature of, for example, about 90C10C.

This heating system comprises a heat exchanger 11 producing hot water M by heat exchange between heating fluid S and cold water C, a heating fluid passage 13 for guiding the heating fluid S from a heating fluid supply source VA to the heat exchanger 11, a cold water passage 12 for guiding the cold water C from a water supply source WA to the heat exchanger 11, a pressure-regulating valve 17A disposed in the heating fluid passage 13 for regulating a pressure of the heating fluid S supplied to the heat exchanger 11, and a hot water lead-out passage 14 for leading out the hot water M from the heat exchanger 11. The pressure regulating valve 17A is constituted to increase a valve opening according to the lowering of the pressure of the heating fluid S at its downstream.