PURPOSE: To obtain a heat transfer tube having large heat transfer promoting effect, by a method wherein a flat plate, provided with a plurality of protuberances, is inserted into a round tube and is fixed on the center line of the section of the round tube.

CONSTITUTION: A flat plate 20 is made of a material having a high heat transfer coefficient and a rigidity. The flat plate 20 is provided with a plurality of protuberances 21 continuously in the lengthwise direction thereof so that the protuberances are made by cutting and raising a part of the flat plate 20 so as to be slanted with respect to the surface of the flat place. The protuberance 21 is cut and raised so that the end face 22 thereof becomes orthogonal to the lengthwise direction of the flat plate and is slanted alternately with respect to the surfaces C, D of the flat plate 20. The flat plate 20, thus prepared, is warped so as to have the shape of bow by utilizing the elasticity thereof and is inserted into a round tube 10 to pinch it between grooves 11, 12 and fix it on the center line of the section of the tube. When fluid flows through a heat transfer tube, thus constituted, the fluid near the center of the tube, whose heat was difficult to be transferred so far, can transfer the heat thereof to the round tube through the flat plate, therefore, an effect, equal to the case where the heat transfer area in the tube is increased by the surface area of the flat tube may be obtained.