A secondary heat exchanger B for recovering latent heat from combustion gas includes water tubes 5 each of which is so inclined that a first end 50a is positioned lower than a second end 50b. Therefore, in draining water from each of the water tubes 5, water can be caused to flow smoothly into a water-inflow and hot-water-outflow header 6A connected to the first ends 50a. The secondary heat exchanger B includes a casing 7 which includes an upper wall 70a and a bottom wall 70b which are so inclined that the inner surfaces thereof extend generally in parallel with the water tubes 5, and the upper and the lower gaps 79a and 79b have constant widths s2 and s3. Therefore, the amount of heat recovery from the combustion gas passing through the gaps 79a and 79b can be increased.