A batch mixer having a barrel which forms a cylindrical sealed chamber having a section consisting of two mutually communicated circles, and two rotors housed in the chamber to mix a mixing material by rotation, at least one of the inner wall of the barrel and the surface of the rotor having a plurality of recessed parts or protruding parts. The recessed parts or protruding parts are formed of grooves or projections extended in the direction crossing to the rotating direction of the rotor rotated in the chamber to mix the material. According to this structure, a batch mixer excellent in the controllability of material temperature during mixing can be provided.