The apparatus involved herein comprises a body (1), an element (4) rotatably arranged in the body and a counter knife device (15), which is provided on the body and adapted to cooperate with the rotatable element for disintegration of material. The counter knife device comprises a base part (17) and a cutting member (19) removably connected to the base part by means of connecting members (18). The body forms a casing (20), whereas the counter knife device (15) is adjustably arranged in an opening (21) in the casing. The connecting members (18) for the cutting member are accessible from the exterior of the body without need for removing the counter knife device from the opening in the casing. A bearing member (25) is arranged adjustable into bearing abutment against the counter knife device.