PURPOSE: To very easily obtain coiled tube having circular cross-section and consequently excellent flow characteristics by a method wherein a straight tube, the cross-sectional form of which is formed elliptically in advance, is wound in such a manner that the surface in the direction of the major axis of an ellipse contacts with the surface of a cylindrical mandrel.

CONSTITUTION: A straight tube 1 having elliptic cross-sectional form is wound around a mandrel in such a manner that the surface in the direction of the major axis of an ellipse contacts with the surface of the cylindrical mandrel by means of a proper means such as by rotating the mandrel or the like. Because the winding of the tube 1 around said mandrel produces the force of flatten the tube 1, the force contracts the major axis of an ellipse, resulting in turning the cross-sectional form into a circle. Accordingly, when the cross-sectional form of the tube 1 is made elliptically, said elliptic form must have the form, which turns into a circular form when the tube is wound in coils.