of stock the respective steps having a tooth shape similar to spur gear the partition 28 may be omitted to have a single accepted chamber in this case a single accept stock outlet is provided next referring to fig3 the mode of operation of the apparatus for screening waste paper pulp according to the first embodiment of the invention will be described the waste paper stock pulp slurry containing undefibered waste paper is introduced through the stock inlet 3 into the inlet chamber 13 and is sent to the primary screening section 9 inside the primary screen plate 18 so that a high quality stock passes through the plate 18 into the primary accept stock chamber 14 and is sent to a next stage through the primary accept stock outlet 4 the waste paper pulp slurry which did not pass through the plate 18 in the primary screening section 9 is sent to a secondary screening section 10 is diluted with dilution water supplied through the dilution openings 25 of the rotor 2 and undergoes screening high quality stock passes through a secondary screen plate 19 into the secondary accepted stock chamber 15 and is sent to a next stage through the secondary accepted stock outlet 5 the rejected stock which did not pass through the screen plate 19 at the secondary screening section 10 includes contaminants such as plastics to be removed and undefibered waste paper which are accumulated and is sent to the defibering section 11 where the undefibered waste paper is defibered by the action of agitated turbulence and at the same time pressure is increased by pumping action of the defibering section 11 in this case contaminants.