A screw extruder comprises an extruder body (4) with a bore (4H), and a screw (5) rotatably disposed in the bore (4H). The screw (5) comprises a screw shaft (7) and two screw flights (8 and 9) disposed in the form of a double helix. The screw flight (9) is substantially continuous, but the screw flight (8) is provided with large interruptions (11) within which a plurality of additional flight pieces (13) are disposed to change the number of passages repeatedly. Mixing/kneading pins (15) protruding into the bore (4H) can be provided. Preferably, the paths (17) for the mixing/kneading pins (15) are positioned within the interruptions (11) and on the upstream side and downstream side of the additional flight pieces (13).