A kneader (N) includes a casing (3) with a papermaking material supply opening (3a) and a papermaking material outlet (3b) located below the papermaking material supply opening; at least first, second, third and fourth rotational shafts (4-7) provided inside the casing; first blades (8) attached to the first rotational shaft (4); second blades (9) attached to the second rotational shaft (5); third blades (10) attached to the third rotational shaft (6); and fourth blades (11) attached to the fourth rotational shaft (7). The first rotational shaft and the second rotational shaft are located at the same height and are arranged parallel to each other, and the third rotational shaft and the fourth rotational shaft are located at the same height and are arranged parallel to each other. The third rotational shaft is located under the first rotational shaft and the fourth rotational shaft is located under the second rotational shaft, respectively.