Extruder

The cross-sectional area of the inside of a pair of cylinders 21, 22 in the direction orthogonal to the direction from a supply region AR1 to a kneading region AR2 is greater in the kneading region AR2 (cross-sectional area S2) than in the supply region AR1 (cross-sectional area S1). An extruder 10 can be enlarged thereby, that is, the residence time of a resin raw material (resin pellets PR, molten resin MR) can be lengthened, and the processing capacity can be increased without making the pair of cylinders 21, 22 and the screw 50 longer and without increasing the diameter of the pair of cylinders 21, 22 and the screw 50. Resin raw materials that require a longer residence time can thus be handled easily, making it possible to use the extruder 10 for a wide range of melt kneading applications.