A head box has a preferably fixed flow-carrying wall (11) and a movable flow-carrying wall (13) defining a nozzle-like stock conduit (14) and an outlet slot (15). The movable wall is provided with an adjustable profile bar (35) at the stock outlet slot. For coarse adjustment of the slot width, two lifting devices (21) act on the movable flow-carrying wall. Fine adjustment of the slot width is performed by adjustment spindles (36) acting on the profile bar.To improve adjustment of the stock outlet slot by the movable wall and to use the profile bar adjustment to perform very small corrections to the slot width, a front end section (24) of the movable wall located in the region of the stock outlet slot is flexibly constructed with respect to the remaining part of the wall, and a plurality of adjustment units (33) are spaced across the machine width and act on the flexible end section directly in front of the stock outlet slot.Thereby the movable wall (33) can be adapted with relatively high accuracy to the contour of the fixed wall (11) on the outlet slot side by the adjustment units (33) acting on the flexible end section (24). The immersion depth of the profile bar (35) into the stock conduit is therefore largely constant over the machine width. Cross currents and variable velocities of the stock flow are avoided and the paper's properties are improved.