PURPOSE: To improve productivity of a finned sheathed heater, by fitting fins on the outer surface of a metal pipe, which is composed of a base metal of aluminum alloy inside and a filler metal of aluminum brazing outside, and heating the assembly at a temperature where the brazing filler metal is soluble.

CONSTITUTION: After removing oil or dirt on the welding surface of fins 5 and the outside a metal pipe 1 by means of a mechanical method such as wire brushes or chemical method such as 5--10 caustic soda solution, the surfaces are thinly coated with pasty flux composed of halogenides of alkali metal and alkaline earth metal. Then the fins 5 are fitted to the metal pipe 1 by means of jigs, and the assembly is heated up to a temperature lower than those of melting the base metal 7 of the pipe and aluminum alloy of the fines 5 ahigher than that of melting the brazing filler metal 8 of the metal pipe 1, to melt the filler metal 8 for brazing. This manufacturing method can improve the productively of the finned sheathed heater