To always keep a satisfactory response speed for feedback control regardless of the water volume change and thereby, to always secure the coincidence between the discharging hot water temperature and a set temperature by detecting the volume of water flowing to a heat exchanger and controlling the proportional and integral gains in response to the detected volume of water.

The deviation between the set temperature TS and the discharging hot water temperature TH is first calculated at an arithmetic part 22. Then, the deviation is multiplied by the proportional gain KP at an amplification part 26, so that the proportional correction value P is obtained. The change amount of the deviation is calculated per unit time at a differentiation part 28 and then multiplied by the differential gain KD at an amplification part 30, so that the differential correction value D is obtained. The deviation is also multiplied by the integral gain KI at an amplification part 32 and added to the hitherto integral correction value at an integration part 34, so that the present integration value I is obtained. Under such conditions, both gains KP and KD change to be inversely proportional to the present supplied water volume Q. The gain KI, however, changes to be proportional to the volume Q. Then, the correction value P, D and I are added together at an addition part 36 and then added to the necessary heating value calculated by a feedforward control operation to be corrected.