Screening apparatus for separating fibrous suspensions, preferably pulp suspensions. The screening apparatus comprises a screen housing (1) with a screen chamber (4), which inward is defined by a rotary tubular screen means (5) with a knob ring (8) located lowermost on the screen means (5), outward is defined by an outer defining surface (6), and downward is defined by a bottom portion (7). Between the knob ring (8) and the slot ring (9) a gap (10) is formed. The screening apparatus further comprises an inlet (11) for fibrous suspension to the screening apparatus, a reject outlet (15) for reject from the screen chamber (4) and an accept outlet (14) for accept from the screening apparatus. For adjusting the gap (10), the slot ring (9) is moveable by means of at least one gap adjustment device (19) in axial direction in relation to the bottom portion (7). Every gap adjustment device (19) is located in the bottom portion (7) and can be operated from the outside of the screening apparatus.