Abstract: An object of the present invention is to diminish the sizes of most disintegrated material effectively, said sizes being determined by the box seats made on the surface of the under roll (7). In a disintegrator (1) having a case (4) with an upper hopper 2 and under exit (3), a pair of upper rolls (5, 6), an under roll (7) located below the upper rolls, and knives (8,9) projecting from inner surface of the case (4) toward the under roll (7) comprising a number of annular grooves (25) formed on the under roll (7), each groove (25) being coupled by a rotative disk cutter (27) at under side of the knives (8, 9). The under roll (7) is provided with polygonal installation seats (13), between adjoining installation seats (13) each attached by a number of cutting edge blocks (14) on a same circumference, said annular grooves (25) are formed between the adjoining installation seats (13).