PURPOSE:To contrive to improve the surface replacement effect and to prevent venting-up from occurring by a structure wherein 2-6 flutes, each of which consists of two threads respectively having specified angles in the axial direction and one group of each of which is a flute open on upstream side and closed on downstream side and the other group of each of which is a flute open on downstream side and closed on upstream side, are provided in such a manner that said two groups of flutes are arranged alternately. CONSTITUTION:Flutes 14, each of which is open on upstream side and closed on downstream side, and flutes 15, each of which is open on downstream side and closed on upstream side, are alternately arranged with no communication between the flutes on the outer peripheral surface of a screw element 13 provided just before the vent port 12 of a screw main body 11. Resin entered the flute 14 from the main body 11 side passes over a thread 16 through a small gap (b) into the flute 15. The angle alpha between the thread 16 and the axis of the main body is set at 25-45 deg., because resin is sent in the flute 14 at all times. Further, because the depth of the flute 15 is made larger than that of the flute 14 or the volume in the flute 15 is larger than that in the flute 14, the resin entered the flute 15 is released from its compressed state, resulting in increasing volumetrically and consequently improving the surface replacement effect. The angle beta between the thread 17 and the axis of the main body is set at 50-75 deg., because the flute 15 itself must have resin conveying capacity. The number of flute is made to be 2-6.