To reliably prevent a combustion gas from flowing into a drain passage even when exhaust air is blocked, in a water seal trap device disposed in the drain passage 8 for draining the drainage generated in a latent heat recovery type heat exchanger disposed in a combustion gas passage, so that the combustion gas does not flow into the drain passage.

A water seal trap device includes a storage container 90 having an inflow port 91 communicating with an upstream-side part 8a of a drain passage 8, a drain port 92 formed on a part in the storage container 90 positioned above the inflow port 91 by a prescribed height, and communicating with a downstream-side part 8b of the drain passage 8, a valve cover 93 mounted on the storage container 90, and displaceable between a closing side for closing the drain port 92 and an opening side for opening the drain port 92, a back pressure chamber 94 opposed to a face at a side opposite to a face opposed to the drain port 92, of the valve cover 93, and a back pressure passage 95 for pressing the valve cover 93 to the closing side by introducing a gas pressure in the combustion gas passage into the back pressure chamber 94.