Provided are a heat exchanger that does not impede downsizing and removes water in the heat absorbing pipe adequately with a simple configuration even when the reduction in diameter of the heat absorbing pipe is made. A heat exchanger 5 in which heat absorbing pipes 51 are disposed in a multi-tier arrangement within a casing 50 which is the passage of combustion exhaust gas, both pipe ends 511,512 of each of the heat absorbing pipes 51 are connected respectively to two headers 54, 55 provided on a side plate 52 of the casing 50, and water introduced from an external pipe 63 to each of the heat absorbing pipes 51 through the header 54 is heat-exchanged and heated by combustion exhaust gas. The pipe ends 511, 512 of the heat absorbing pipes 51 are arranged at a predetermined vertical interval. A drainage plate 56 for forming a drainage passage through which the water that has reached the pipe end openings 51A of respective heat absorbing pipes 51 is removed during drainage operation for the heat absorbing pipes 51, is disposed in the header 54 disposed on a lower side of the heat absorbing pipes 51 so as to face a number of the pipe end openings 51A vertically arranged in a state of continuous.