Abstract: The present invention relates to a washing apparatus for washing of cellulose pulp comprising: a rotatable drum (2), a stationary support (14), a stationary cylindrical casing (18), that encloses the drum, whereby an annular space (20) is defined between the casing and the drum, a number of seals (22) that are arranged on the casing and that seals between the casing and the drum, such that the annular space is divided in a formation zone (F), at least one washing zone (T1, T2), and a discharge zone (U), and at least a reinforcing frame rib (24), that is attached to the casing. The reinforcing frame rib (24) extends around the entire casing (18) for fixing the casing in a predetermined form, whereby detrimental deformation of the casing (18), when washing the paper pulp at overpressure, is prevented.