A kneader/stirrer includes two rotary shafts extending parallel to each other in the processing chamber, and first and second regular triangular impellers mounted to the respective rotary shafts. The first impellers are located at the same axial positions as the respective first impellers. The impellers are fixed to the respective rotary shafts at points offset from the centers of the respective impellers in a predetermined direction by a predetermined distance such that when the two rotary shafts rotate in the same direction at the same speed, each pair of first and second impellers are eccentrically pivoted, with one apex of one of the pair of impellers kept in contact with or in close proximity to the contour of the other impeller, thereby scraping off polycondensation resin (material to be processed) adhered to the other impeller.