A water heater comprising: a hot water tank in which hot water is stored; a water supply pipe through which water is supplied to a lower portion of the hot water tank; an entering-water pipe through which the water is made to flow out from the lower portion of the hot water tank; a heating device for heating the water sent from the entering-water pipe; an outgoing-hot water pipe through which the water heated by the heating device is supplied to an upper portion of the hot water tank; a water quality adjusting unit placed around the entering-water pipe and comprising a water quality adjusting portion for reforming the inflow water into water having quality in which scale is less prone to be generated, wherein the entering-water pipe or the water supply pipe includes a first closing valve which is placed upstream of the water quality adjusting portion and which closes a flow path, and a second closing valve which is placed downstream of the water quality adjusting portion and which closes a flow path; characterized in that the water quality adjusting portion comprises a casing charged with scale inhibitors, an inflow port located at a vertically lower portion of the casing and an outflow port located at a vertically upper portion of the casing such that water flowing into the water quality adjusting portion from the inflow port and out from the outflow port passes the entire casing with the scale inhibitors, wherein the water quality adjusting unit is connected to the entering-water pipe through a first connecting portion and a second connecting portion, wherein the first connecting portion is provided upstream of the water quality adjusting portion, and wherein the second connecting portion is provided downstream of the water quality adjusting portion; wherein, by connecting the first and second connecting portions (47a, 47b) and the entering-water pipe to each other, the water quality adjusting unit can be attached to and detached from the entering-water pipe; wherein the drainage passage provides a drainage valve; and the drainage valve is located between the first closing valve and the second closing valve. a hot water tank in which hot water is stored; a water supply pipe through which water is supplied to a lower portion of the hot water tank; an entering-water pipe through which the water is made to flow out from the lower portion of the hot water tank; a heating device for heating the water sent from the entering-water pipe; an outgoing-hot water pipe through which the water heated by the heating device is supplied to an upper portion of the hot water tank; a water quality adjusting unit placed around the entering-water pipe and comprising a water quality adjusting portion for reforming the inflow water into water having quality in which scale is less prone to be generated, wherein the entering-water pipe or the water supply pipe includes a first closing valve which is placed upstream of the water quality adjusting portion and which closes a flow path, and a second closing valve which is placed downstream of the water quality adjusting portion and which closes a flow path; characterized in that the water quality adjusting portion comprises a casing charged with scale inhibitors, an inflow port located at a vertically lower portion of the casing and an outflow port located at a vertically upper portion of the casing such that water flowing into the water quality adjusting portion from the inflow port and out from the outflow port passes the entire casing with the scale inhibitors, wherein the water quality adjusting unit is connected to the entering-water pipe through a first connecting portion and a second connecting portion, wherein the first connecting portion is provided upstream of the water quality adjusting portion, and wherein the second connecting portion is provided downstream of the water quality adjusting portion; wherein, by connecting the first and second connecting portions (47a, 47b) and the entering-water pipe to each other, the water quality adjusting unit can be attached to and detached from the entering-water pipe; wherein the drainage passage provides a drainage valve; and the drainage valve is located between the first closing valve and the second closing valve.