Abstract: In order to enable thickness screening of the chips in a drum screen comprising a rotatable screen drum with screening slits (1) extending in circumferential direction and formed between a plurality of parallel rings (3) arranged one after the other and joined together at a predetermined distance from each other by means of cross pieces (4) extending axially on the inside of the rings (3), the invention suggests that at least a plurality of the rings (3) of the drum screen is provided with a plurality of guide plates (5) arranged along their inner circumference surfaces, substantially parallel to the screening slits (1) between the rings (3) in order to turn long splinters and chip pieces, enabling them to pass through the screening slits (1) intended for that particular chip thickness. The rings (3), as is known per se, consist of tubes or rods having radius cross section and bent to substantially circular shape.