A water heater, comprising: a scale deposition device; a tank; characterised by further comprising: scale deposition means including an anode and a cathode provided in the tank so as to be opposed to each other, and a power supply for applying a voltage between the anode and the cathode, for depositing a scale component dissolved in water retained in the tank; a heat-transfer channel provided at downstream side 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, wherein the scale deposition means comprises oxygen-gas supplying/dissolving means (20, 24; 20, 134) 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, and wherein the tank comprises a partition plate provided to a ceiling of the tank between the anode and the cathode, for preventing the oxygen gas from flowing from the anode side along the ceiling of the tank toward the cathode side and wherein: the oxygen-gas supplying/dissolving means (20, 24) comprises: an ejector mounted to the water feed pipe and connected to the anode side room of the tank; and a return pipe having one end portion connected to the anode side room of the tank so as to lead to a space above the anode, and another end portion connected to the ejector; and wherein the oxygen gas generated at the anode by the application of the voltage is sucked by the ejector through the return pipe. a scale deposition device; a tank; characterised by further comprising: scale deposition means including an anode and a cathode provided in the tank so as to be opposed to each other, and a power supply for applying a voltage between the anode and the cathode, for depositing a scale component dissolved in water retained in the tank; a heat-transfer channel provided at downstream side 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, wherein the scale deposition means comprises oxygen-gas supplying/dissolving means (20, 24; 20, 134) 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, and wherein the tank comprises a partition plate provided to a ceiling of the tank between the anode and the cathode, for preventing the oxygen gas from flowing from the anode side along the ceiling of the tank toward the cathode side and wherein: the oxygen-gas supplying/dissolving means (20, 24) comprises: an ejector mounted to the water feed pipe and connected to the anode side room of the tank; and a return pipe having one end portion connected to the anode side room of the tank so as to lead to a space above the anode, and another end portion connected to the ejector; and wherein the oxygen gas generated at the anode by the application of the voltage is sucked by the ejector through the return pipe. scale deposition means including an anode and a cathode provided in the tank so as to be opposed to each other, and a power supply for applying a voltage between the anode and the cathode, for depositing a scale component dissolved in water retained in the tank; a heat-transfer channel provided at downstream side 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, wherein the scale deposition means comprises oxygen-gas supplying/dissolving means (20, 24; 20, 134) 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, and wherein the tank comprises a partition plate provided to a ceiling of the tank between the anode and the cathode, for preventing the oxygen gas from flowing from the anode side along the ceiling of the tank toward the cathode side and wherein: an ejector mounted to the water feed pipe and connected to the anode side room of the tank; and a return pipe having one end portion connected to the anode side room of the tank so as to lead to a space above the anode, and another end portion connected to the ejector; and wherein the oxygen gas generated at the anode by the application of the voltage is sucked by the ejector through the return pipe.