To securely prevent deposition of scale and prevent decline in the capacity of a water heater without increasing the size of and complicating the water heater.

The water heater 100 includes: a supply port 160 for supplying water; an injection means 130 for injecting carbon dioxide to the supplied water; and a heater 116 for generating hot water by making the water dissolved with carbon dioxide by injection it flow as a two-phase flow with microbubbles of carbon dioxide and heating the water. The injection means 130 is provided upstream of the heater 116.