established value this lowers the efficiency of defibration for the stock material and eliminates excessive defibration because the stock material passes through at a greater speed in the defibrating unit 30 and the number of times where the rotating blade hits against the stock material is reduced this reduces short fibers and makes it possible to product sheets with which strength has been ensured next a first example shall be described fig7 is a drawing illustrating a state of defibration as in the first example more specifically fig7 illustrates the circumstances of defibrated material whether or not short fibers occur and whether or not fiber clumping occurs in cases where four different kinds of changes are made to the rotational speed of the rotating blade of the defibrating unit 30 in accordance with four different kinds of water content ratio contained in the stock material being fed in in the drawing the symbol u201c u25ef u201d is indicative of the fact that neither short fibers nor fiber clumping occurred and favorable paper was obtained as illustrated in fig7 favorable paper was obtained in cases where the water content ratio of the stock material was low and the rotational speed of the rotating blade of the defibrating unit 30 was low in turn defibration became excessive and short fibers occurred in cases where the water content ratio of the stock material was low and the rotational speed of the rotating blade of the defibrating unit 30 was high favorable paper was further obtained in cases where the water content ratio of the stock material was high and the rotational speed of the rotating blade of the defibrating unit 30 was high.