Abstract: One heat transfer tube for an absorption refrigerating machine of the present invention has a plurality of grooves formed on the circumferential surface of a tube at uniformly angular intervals to extend continuously or discontinuously in the length direction of the tube, wherein the width and/or depth of each groove gently varies in the length direction of the groove, and the height of each ridge between the mutually adjacent grooves gently varies from the axial tube center in the length direction of the ridge. Another heat transfer tube of the present invention has a large number of concave portions formed in rows on the circumferential surface of the tube at predetermined angular intervals and each having a gently down-grade surface extending in the tube length direction to gradually get closer to the axial tube center and a gently up-grade surface extending continuously from the down-grade surface in the tube length direction to gradually become more distant from the axial tube center.