A heat exchanger configured to heat water by heat exchange with combustion exhaust gas ejected from a burner, the heat exchanger comprising: a case body; a water supply pipe supplying the water to the heat exchanger; a hot-water supply pipe supplying heated water from the heat exchanger to a hot-water supplying terminal; a plurality of heat-transfer tubes configured to flow the water therethrough and arranged in at least a vertical direction inside the case body; a header configured to communicate open ends of the plurality of heat-transfer tubes arranged in the vertical direction, the header being provided in a side wall outer surface of the case body, and having a header body to which the open ends of the plurality of heat-transfer tubes are connected, and a header cover joined to the header body; a drainage member disposed at at least one of the open ends protruding into an internal space of the header, the drainage member having a flow guide portion including an insertion portion configured to be inserted into the heat-transfer tube from the open end along a lower inner wall of the heat transfer tube, and a projecting portion provided continuously to the insertion portion so as to project toward an outside of the heat-transfer tube from the open end, and further being bent and extended downward from the insertion portion; and a flow guide space configured to guide the water inside the heat-transfer tube to the outside of the heat-transfer tube during the drainage work, wherein the insertion portion is inserted into the heat-transfer tube from the open end such that the flow guide space being formed continuously between the insertion portion and a lower inner wall of the heat-transfer tube, and between the projecting portion and the open end of the heat-transfer tube. a case body; a water supply pipe supplying the water to the heat exchanger; a hot-water supply pipe supplying heated water from the heat exchanger to a hot-water supplying terminal; a plurality of heat-transfer tubes configured to flow the water therethrough and arranged in at least a vertical direction inside the case body; a header configured to communicate open ends of the plurality of heat-transfer tubes arranged in the vertical direction, the header being provided in a side wall outer surface of the case body, and having a header body to which the open ends of the plurality of heat-transfer tubes are connected, and a header cover joined to the header body; a drainage member disposed at at least one of the open ends protruding into an internal space of the header, the drainage member having a flow guide portion including an insertion portion configured to be inserted into the heat-transfer tube from the open end along a lower inner wall of the heat transfer tube, and a projecting portion provided continuously to the insertion portion so as to project toward an outside of the heat-transfer tube from the open end, and further being bent and extended downward from the insertion portion; and a flow guide space configured to guide the water inside the heat-transfer tube to the outside of the heat-transfer tube during the drainage work, wherein the insertion portion is inserted into the heat-transfer tube from the open end such that the flow guide space being formed continuously between the insertion portion and a lower inner wall of the heat-transfer tube, and between the projecting portion and the open end of the heat-transfer tube.