PURPOSE: To manufacture a smooth surface tube mentioned above by expanding the diameter of a plastic tube under the force in a radial direction of a spring on a diameter expander provided on a mandrel.

CONSTITUTION: A diameter expander 2 with a bevel-shaped head 2a provided on a mandrel body 1a in a longitudinal direction is kept in a contracted diameter state resisting against the force of a spring 4. Then this mandrel for diameter expansion 1 is inserted into a plastic tube 6. Following this procedure, the tube 6 is heated beyond its softening temperature level so that its rigidity may be lost and the tube 6 is expanded in diameter by protruding the diameter expander 2 outwards under the force of the spring 4. Afterward, the tube 6 is cooled and the mandrel 1 is pulled out.