In a two-shaft extruder, a pair of rotor shafts (1) is rotatably arranged such that the closer to the head end of the pair, the smaller the distance between the pair. The pair of rotor shafts (1) is respectively provided with screw blades (2) mounted on the head end side of the pair. The rotor shafts (1) are adapted such that one screw blade (2a) enters gaps of the other screw blade (2b). Drive devices (M) for rotationally driving the pair of rotor shafts (1) are mounted on the respective base ends of the pair. The extruder has a screw blade contact prevention mechanism (SS) which, when the pair of rotor shafts (1) is driven, prevents the pair from having a phase difference greater than a predetermined level.