The sieve apparatus according to the invention serves to treat a contaminated fibrous stock suspension, as is produced, for example, in the processing of waste paper. It has a housing (1) having a sieve (3) which separates an accepted stock chamber (2) from the housing (1). A rotor (4) can be moved closely along the sieve (3), as a result of which the latter is kept free and, moreover, a rotational flow is generated. The housing (1) has a main part (7) which is, in particular, cylindrical and the centre of which lies at an oblique angle (Alpha) with respect to the axis (12) of the rotor (4) which lies between 1 and 60 DEG , preferably between 5 und 25 DEG . The typical use position of this apparatus is the standing arrangement.