An extruder capable of obtaining a treatment volume equal to that of the conventional art without enlarging a screw diameter is provided. The extruder includes screw shafts, a drive unit, and a barrel, in which three or more screw shafts having the same diameter and the same root diameter in a waveform shape are engaged with each other and arranged in parallel to each other horizontally or nearly horizontally, while a distance between axial centers is made equal. According to the above configuration, the same treatment volume as that of a conventional extruder can be handled by screw shafts of smaller diameter as compared with that of the conventional art. By using the screw shafts of the smaller diameter as compared with the conventional art, temperature irregularities of an extrusion material in screw grooves, leakage of the extrusion material from the screw shafts, an installation area of the extruder, and the like conventional trouble of various kinds can be eliminated.