A natural circulation-type hot water storage heater for storing hot water obtained from a water storage tank (as water or hot water) at the bottom through a first circulation pipe, a heat exchanger and a second circulation pipe into a tank. A thermovalve is mounted within the second circulation pipe on the downstream side of the heat exchanger to vary its opening area in response to the hot water temperature to thereby substantially maintain constant the temperature of the hot water from the heat exchanger. As a result, the temperature distribution at the time the water is boiling in the water storage tank is rendered substantially constant instantaneously so as to increase the effective hot water storage amount. In addition, the hot water temperature is held constant without fluctuation regardless of the repetitive ON and OFF operation of a burner, and will not fluctuate even if the burner is turned ON when hot water is supplied. Instead, the time during which the hot water is continuously supplied at constant temperature is prolonged to provide an effective hot water storage amount which is greater than the inside volume of the water storage tank.