tank 15 before being supplied to the user thereby obtaining the water having the temperature substantially close to the user set temperature accordingly since a comparatively large amount of the temporary undershoot occurring due to the increase of the usage of the hot water is absorbed in the water tank the hot water can be supplied to the user without a remarkable change of the temperature of the hot water additionally when the usage of the hot water is increased a large amount of the overshoot which occurs by rapidly increasing the heating power of the burner in order to alleviate the undershoot may also be absorbed in the water tank fig1 illustrates a change of the temperature of the hot water in the water heater 100 according to the present invention when the user increases the usage of the hot water as shown in the graph of fig1 the overshoot and the undershoot are remarkably decreased in comparison with the prior art water heater even when the user decreases the usage of the hot water the overshoot in which the temperature of the water discharged from the heat exchanger 13 temporarily increases occurs however according to the water heater of the present invention since the water having a comparatively high temperature that is discharged from the heat exchanger is mixed with the water stored in the water tank 15 the comparatively large portion of the overshoot is absorbed in the water tank 15 accordingly the amount of the overshoot is remarkably decreased such that the hot water can be supplied to the user without the remarkable change in the temperature of the hot water additionally when the usage of the hot water decreases a comparatively large amount of the undershoot which occurs by rapidly decreasing.