PURPOSE: To prevent a spiral flow high in motion resin viscosity from deteriorating by a method wherein a cut starting point is provided at a position 2mm or deeper in a spiral groove and a distance of the cut starting point to the cut terminating point is made 50mm or less.

CONSTITUTION: The molten resin supplied from an extruder appears at each opening 1 on the surface made when flow is branched from the axis part to four radial directions and comes to each cut part 3 after turning 1.25 rounds in spiral flow along each spiral groove 2. The cut part 3 ends suddenly. The cut starting point P is given at a position 2mm or deeper preferably 3mm or deeper in each spiral groove and 50mm or less preferably 30mm or less is given to a distance from the cut starting point P1 to the cut terminating point P2. In this manner, the spiral flow can be prevented from deteriorating when molten resin is high in viscosity because pressure changes suddenly at each cut part 3 and pressure loss decreases or by other reasons.