To reduce the load of a fibrillating crusher by eliminating overgrinding when a woody material is defibrated and ground.

A large mesh screen 11 is arranged to the crushed matter discharge port 10 of the defibrating crusher 1 and the crushed matter C discharged from the discharge port 10 through the screen 11 is introduced into a cyclone 21. The fine crushed matter (p) sorted by the cyclone 21 is collected and the coarse crushed matter P is returned to the defibrating crusher 1 to be crushed again. By this system, the overgrinding of the woody material into a size smaller than a target size due to the defibrating crusher 1 is prevented effectively.