PURPOSE: To recover discharged gas heat with water in an over-head water tank by a method wherein water in the overhead water tank is sprayed into a latent heat recovering device and this water is returned back into the overhead water tank.

CONSTITUTION: Water in an overhead water tank 3 is supplied from a latent heat recovering line 12 to a spray device 9 and sprayed from the spray device 9 toward a latent heat recovering device 10. In turn, discharged gas passed from a discharged gas passage 6 through a discharged gas taking line 16 into a latent heat recovering device 8 passes through the latent heat recovering device 10, supplies latent heat to the sprayed water during its lifting stage, the discharged gas passes from a discharged gas recovering port 17 through a return line 18 and then the gas is returned back into the discharged gas passage 6. Water heated by the latent heat recovering device 10 is once accumulated at a lower hot water storing part 11 and returned back from a latent heat recovering return line 13 with a force of a pump 14 to the overhead water tank 3. Since the discharged gas heat can be recovered with water in the overhead water tank 3, it can lead to an energy saving. Water is directly sprayed against the discharged gas to cause acid substances contained in the discharged gas to be washed away and an interior part of a chimney is prevented from being decayed.