PURPOSE: To improve thermal conductivity and to prevent the generation of wear powder by a method wherein a coil is forced into high pressure contact with the inner surface of a heat-transfer pipe.

CONSTITUTION: Formation of a heat-transfer pipe P for a heat-exchanger is such that a coil 7 having elasticity and an outside diameter larger than the inside diameter of the heat-transfer pipe is inserted in heat-transfer pipe 2 by throttling the coil against a spring in such a manner that the outside diameter thereof is reduced. By releasing the throttling, the coil is brought into pressure contact with the inner surface of the heat transfer pipe 2 through the repulsive force of elasticity.