One embodiment provides a twin-screw extruder which is used to knead materials and has excellent productivity. The twin-screw extruder includes screws. The screws include double thread screws in which first screw portions engage with each other. Ratio D1/C of external diameter D1 of the first screw portion to a shaft distance C ranges from 1.25 to 1.40. The screws further include triple thread screws in which third screw portions engage with each other. Ratio D3/C of external diameter D3 of the third screw portion to the shaft distance C ranges from 1.05 to 1.10. External diameter D2 of a second screw portion decreases gradually from the first screw portion toward the third screw portion.