A method and an apparatus for extruding material, the material being extruded with an apparatus comprising at least one rotatable rotor and at least one stator, the rotor and the stator comprising grooves, whereby the grooves make the material move through the apparatus when the rotor is rotated. The rotor and stator surfaces facing each other have a wavelike cross-section, and the grooves are so arranged that at the ridge of a wave the depth of a groove reaches its maximum, and at the bottom of a wave the depth reaches its minimum. The bottom of the wave in the rotor is at the ridge of the wave in the stator and vice versa. This makes the material move alternately from a rotor groove to a stator groove, and back.