A headbox for a paper machine, with a main chamber through the lower part of which flows fiber suspension in the longitudinal machine direction. The main chamber has on the upstream end a first perforated roll and a machine-wide feed channel as well as on the downstream end a second perforated roll and a machine-wide nozzle type outlet channel. Provided between the perforated rolls is a vertical partition which is vertically adjustable, creating between the bottom of the main chamber and the partition a machine-wide channel section which is variable in its height. The downstream outside surface of the partition has a plurality of abutted curved surfaces, in a fashion such that the clearance of the channel section increases in the direction of flow.