A cracking tube (50) for use in thermal cracking furnaces for producing ethylene or the like has fins (1) formed on an inner surface thereof and inclined with respect to an axis of the tube for stirring a fluid inside the tube. The fins are arranged discretely on one or a plurality of helical loci, and the tube inner surface has regions (ZB) wherein no fins are present over the entire axial length of the tube from one axial end of the tube to the other axial end thereof.