PURPOSE: To increase the convection and radiation heat transfer without accompanying a large cost increase by inserting radiation plates along gaps between heating pipes of a dual pipe structure heated by a high temperature gas, at the length part at which fins of outer pipe of the heating pipe are provided.

CONSTITUTION: The cross sectional contor of a finned heating pipe 21 is made into a regular hexagonal shape. A thin plate is inserted into the gap of the heating pipe and used as an oblong flat type radiation plate 31 the width of which is equal to one side of the regular hexagon or a trough-shaped oblong plate the length of one side of which is equal to one side of the regular hexagon and which is bent by 120°. It is desirable that the gap of the finned heating pipe 21 be assumed sufficiently in consideration of the dimension of the plate, and deformation of the plate and heating pipe. Two types of radiation plates 31 and 32 are inserted into the gaps of the heating pipe so that these plates are adapted to the gaps of the heating pipe, and a latch is inserted into the gap of the heating pipe thereby to prevent them from falling downwardly. The radiation plate 31 or 32 is made of a thin metal or a heat resisting material such as ceramics.