A water-and-hot-water mixing device includes a mixing valve unit which mixes hot water and water and discharges mixed water therefrom, an operation part which sets a predetermined temperature, a mixed water thermistor which detects a temperature of mixed water, a hot-water thermistor which detects a temperature of hot water supplied to the mixing valve unit, and a controller which performs a feedback control of a discharge water temperature by controlling the mixing valve unit. The controller starts the feedback control when the controller determines that a change amount of the temperature of the hot water per unit time detected by the hot-water thermistor is not more than a fixed value and the temperature of the hot water detected by the hot-water thermistor is not lower than the predetermined temperature.