A water heater, which allows efficient deposition of a scale component dissolved in water so as to reduce adhesion of the scale to a heat-transfer channel. The water heater includes: scale deposition device including a tank, an anode and a cathode provided in the tank so as to be opposed to each other, and a first power supply for applying a voltage between the anode and the cathode, the scale deposition device depositing a scale component dissolved in water retained in the tank; a heat-transfer channel provided at downstream of the scale deposition device, for guiding the water; and a heat source for heating the water by heat exchange in the heat-transfer channel, in which the scale deposition device includes oxygen-gas supplying/dissolving means for supplying an oxygen gas to a water feed pipe for feeding the water to the tank and dissolving the oxygen gas in the water.