PURPOSE: To enable complete combustion and suppress issuing of harmful gas, covering a combustion chamber with wall formed by water tubes, installing smoke tube in the water tube, installing smoke hole in a part of combustion chamber wall around the inlet of smoke hole, increa sing heating capacity.

CONSTITUTION: When the central burner 11 is burned, inside of water tube 3 is heated by radiant heat and convection heat, combustion gas blows into lower inlet 20 and smoke hole 22 separately, one flow through the smoke tube 19, heating water in the water tube 3 from inside, flowing into upper end of smoke tube 7 from outlet 21, other flow rises the smoke tube 7, heating outer area of water tube 3 and inside area of water tube 4, meeting with combustion gas from outlet 21 at the upper end, flowing into the smoke tube 23 from the inlet 24, heating the water in the water tube 4 from inside, flowing into the smoke tube 9 from outlet 25, heating outside area of the water tube 4, reaching the smoke tube 10 to exhaust. Thus, heating surface is increased, thermal efficiency is improved, harmful gas does issue by complete combustion.