PURPOSE: To improve tear strength and blocking resistance of a tube by moving the freely rotatable disk type mold having periphery of an uneven pattern relatively to the tube extruded and contacting the uneven part of the mold to the outer periphery of the tube.

CONSTITUTION: The gel state tube 19 is formed by continuous extrusion through the die 2 of synthetic resin mixed and melted in an extruder 1 and inflated by air from the air orifice 3. When the tube 19 reaches to the inside of the assembly of rotating molds, the protruded part 13a at the periphery of the mold 13 contacts to the peripheral face of the tube 19, however the tube 19 moves upward constantly, then the locus of contacting place of the protruded part 13a to the periphery of the tube 19 is formed helically. Meanwhile, the second ring plate 11 rotates in reverse direction against to the first ring plate 9, the locus of contacting place of the periphery 13a of the mold 13 to the periphery of the tube 19 becomes a reverse direction helix against the case of said ring plate 9.