PURPOSE: To continuously manufacture a homogeneous rubber-based mixture by inhibiting the remarkable increase of the pressure of a material in the cylin der jacket-shaped annular clearance and using a twin-screw extruder.

CONSTITUTION: A twin-screw extruder comprises two screws 8, 9 engaged with each other. A recycle passage or a slit 13 is communicated with a cylinder jacket-shaped annular clearance 14 from an extrusion chamber 12. An upstream side of the annular clearance 14 is connected with an inlet side of the extrusion chamber 12 through the recycle passage or a slit 17. A filler, that is, carbon black is poured into the rubber pulped and heated in advance in a filling hopper 3, and primarily mixed with the material flow recycled through the passage 17. The pressure is measured by a pressure measuring device 7, and a size of a free passage cross area of an opening 18 is controlled through a controller 6 connected with a shutter 5. Whereby the sufficient homogeneity corresponding to the desired homogeneity can be obtained.