Abstract: A plurality of protrusions 32 are formed in a longitudinal direction on the outer surface of a heat transmission pipe, a convex portion 33 and a concave portion 34 adjacent thereto of each of the protrusions 32 are each in the form of a curved surface, the curvature radius of the concave portion 34 is made larger than the curvature radius of the convex portion 33, and a groove 34A is formed in the bottom of the concave portion so that the absorption solution dropping onto the heat transmission pipe 31 flows from the concave portion 34 over the convex portion 33 to the next concave portion 34 smoothly, the shifting of the absorption solution in the concave portion 34 is carried out smoothly, and Marangoni convections generated in the convex portion 33 and the concave portion 34 interfere with each other.