An object is to reduce a weight of a raw material to be used for manufacturing a screw shaft by combining a tapered section and a tapered ring as a structure of a flange section. A screw shaft structure for a twin screw extruder includes a tapered section (20) provided between a ground section (6) and a screw piece inserting section (8), and a tapered ring (21) fitted onto the tapered section (20), the tapered ring (21) serving as a flange section.
