PURPOSE: To improve the strength of a gap generated by the deviation of an inner fin in a tube while restraining the increase of a weight.

CONSTITUTION: The thickness of both ends 12 of an inner fin, inserted into a tube 4 formed so as to have a flat and long-circular sectional configuration, in a direction orthogonal to the flow direction of heat medium which flows through a heat medium passage 10 is formed so as to be thicker than the thickness of the central part 13 of the inner fin. The strength of both ends 12 of the inner fin 11 is improved without thickening the thickness of the whole of the inner fin 11.