PURPOSE: To reduce a noise of operation by a method wherein a controlling device for supplying and controlling combustible gas and a combustion chamber having a heat exchanger are provided and a discharged gas passage is constituted by a plurality of passages having a different passage length.

CONSTITUTION: A burner 3 in which air and combustible gas supplied from a blower 1 and a gas controlling device 10 are mixed to each other to ignite them, a combustion chamber 5 having a heat exchanger 4 for transmitting heat generated in case of combustion, discharged gas passages 8, 9 and discharging ports 6 and 7 for use in discharging the discharged gas are provided. The two discharging passages are mounted such that each of them has a different passage length. Each of the discharging ports is arranged at a short side direction of the discharging port. Noise generated by combustion passes through the discharging passage 8 and the discharging passage 9 and then is radiated from the discharging port 6 and the discharging port 7. The radiated noise shows a reduced sound pressure by a fact that phases of sound waves caused by a difference in passage lengths of the discharging passages are reversed in their positive and negative values and compensated to each other. Since an arrangement in which the discharging ports 7 are arranged in a short side direction of the discharging port 6 enables a distance between the discharging ports 6 and 7 to be shortened, it is possible to increase this effect even for a noise of low frequency.