Abstract: A screen device for separating undesirable impurities from a pulp suspension is provided with at least one continuous circular screen slot between a first chamber into which the suspension is conducted and from which undesirable material is removed and a second chamber from which the screen pulp suspension is transported. The slot is formed by two walls which are stationary in relation to each other and a cleaning device such as a doctor or knife bears against the screen slot, the cleaning device and slot being movable in relation to each other for continuous cleaning of the screen slot.