There are provided screw segments 100, 200 that are capable of shearing and kneading feedstock without causing plugs within a cylinder 1 of a screw extruder, and of extruding it from the cylinder 1. The screw segments 100, 200 each comprise a shaft part 111 to be affixed to a screw shaft 7, and a tooth part 112 that protrudes radially outward from the shaft part 111. The tooth part 112 comprises a tooth surface 116 that is formed on the front side relative to the rotation direction of the screw shaft 7, and that is so sloped as to transition gradually forward relative to the rotation direction as it transitions radially outward from the shaft part 111.