PURPOSE: To form the hot-water boiler, easy to assemble, without deteriorating heat efficiency, by a method wherein a combustion gas path, through which the combustion gas passes, is formed between a baffle body and an outer wall, provided outside a water-tube wall, and baffle plates are arranged in the combustion gas path.

CONSTITUTION: A nozzle 6, communicated with a ventilating path 9, and an air injecting port 7 are penetrated through an insulating member 10 and the outer wall 12 so as to be opened in a combustion chamber 4. The baffle body 17, having a tubular configuration and whose upper and lower ends are opened, is supported by an exhaust port lid 19 through hanging fitments 18 so as to be located inside the water-tube wall consisting of water tubes 3. The combustion gas path 20 is formed between the insulating membmer 10 and the baffle body 17 and corrugated baffle plates 21 are arranged in the combustion gas path 20 between water tubes 3. According to this method, the high thermal efficiency of multi-tube type boiler may be maintained and man-hours for assembling may be shortened since the constitution thereof does not include welding between the water tubes and the baffle plates.