The pulp to be washed is fed between the rotating drum (1) and the pressing roll (2) inside it, so that pulp passes along a spiral-formed track around the pressing roll (2), being thereby pressed several times. Washing liquid (7) is fed to the pulp at one or several points along its track of movement, between the pressings and/or at the pressing position. Washing thus takes place partly as a diffusion and partly as a displacement washing process. Washing is efficient and brings the breaking of pulp fibers down to a minimum