To provide a water heater reducing the volume of a heat insulating tank for storing hot water by heat insulation.

When storing water, water is heated to a higher temperature than 100°C at a higher pressure than 1 atmospheric pressure and stored in the heat insulating tank 20. In supplying water, when water temperature in the tank is higher than 100°C, water in the tank and tap water are mixed and discharged, and when the water temperature in the tank is a predetermined temperature or lower, only the water in the tank is discharged without being mixed with tap water. With the water stored at the higher temperature than 100°C, the volume of the heat insulating tank can be reduced in comparison with the case of storing water at a temperature than 100°C.