PROBLEM TO BE SOLVED: To provide a scum crusher that can prevent foreign substances, such as a screen residue, from twisting around a blade when the scum crusher is not operated and used.SOLUTION: A scum crusher 3 comprises: at least one belt-like or string-like blade 11 that is rotated in the vicinity of a liquid surface in a digestion tank body 1 (in a digestion tank 100) or at a position between the liquid surface in the digestion tank body 1 (in the digestion tank 100) and its ceiling part; an electric motor 12; and torque transmission means 13 that transmits the rotation of the electric motor 12. When the scum crusher 3 is not used, the blade 11 is stored inside a cylinder 18 disposed in a gas phase in the digestion tank body 1 (in the digestion tank 100) or evacuated to the outside the digestion tank body 1 (the digestion tank 100).
