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; and 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, wherein the entering-water pipe or the water supply pipe includes a main flow path and an auxiliary flow path which bypasses the main flow path, wherein the main flow path includes a water quality adjusting unit placed around the main flow path and comprising a water quality adjusting portion; wherein the main flow path further 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; wherein the water quality adjusting portion is attached to the main flow path for reforming the inflow water into water having a a quality in which scale is less prone to be generated, and to flow the water out; 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 portion can be attached to and detached from the main flow path, and in that the water quality adjusting unit comprises a drainage passage, wherein said drainage passage is arranged between the first closing valve and the inflow port in the water quality adjusting unit; and comprises a drainage valve between the first closing valve and the second closing valve; wherein the drainage valve is placed on a vertically lower side of the water quality adjusting portion for closing and opening a flow path of the drainage passage. 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; and 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, wherein the entering-water pipe or the water supply pipe includes a main flow path and an auxiliary flow path which bypasses the main flow path, wherein the main flow path includes a water quality adjusting unit placed around the main flow path and comprising a water quality adjusting portion; wherein the main flow path further 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; wherein the water quality adjusting portion is attached to the main flow path for reforming the inflow water into water having a a quality in which scale is less prone to be generated, and to flow the water out; a water quality adjusting unit placed around the main flow path and comprising a water quality adjusting portion; wherein the main flow path further 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; wherein the water quality adjusting portion is attached to the main flow path for reforming the inflow water into water having a a quality in which scale is less prone to be generated, and to flow the water out; 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 portion can be attached to and detached from the main flow path, and in that the water quality adjusting unit comprises a drainage passage, wherein said drainage passage is arranged between the first closing valve and the inflow port in the water quality adjusting unit; and comprises a drainage valve between the first closing valve and the second closing valve; wherein the drainage valve is placed on a vertically lower side of the water quality adjusting portion for closing and opening a flow path of the drainage passage.