PURPOSE:To improve the kneading and mixing effects of thermoplastic resin by a method wherein left and right roots are formed so as to be changed in the same phase but the central root is formed so as to be changed in the reverse phase in case the depth of the root is changed from A to B while at least two pieces of A and B sections of the root are provided within one pitch of the screw. CONSTITUTION:Two places of the most shallow section and the most deep section are existing within one pitch of first flight 11 and the phases of the root sections 15, 16 are reversed. pellets within one pitch of the first flight 11 receive strong compression force as well as sure kneading effect by the wedge effect of the root sections and a cylinder 6 when the depths of the roots 15, 16 are changed from A to B (twice during one pitch) or the depth of the root 14 is changed from B to A (twice during one pitch). The kneaded and molten resin is sent forcibly over a clearance C at the tops of second flight 12 and third flight 13 and flows from the shallow side A to the deep side B whereby the resin in the roots 14, 15, 16 may be kneaded well.