The breast box nozzle comprises a nozzle-type pulp stock duct formed by stream-guiding walls of the same width as the machine. The pulp stock duct is limited at its pulp stock discharge gap by a surface which extends over the width of the machine and is not touched by the pulp stock. Deposits in the area of the breast box nozzle which are caused by spray mists may cause defects in basis weight/cross section of the web being produced.The precipitation of the spray mist on the surface not touched by the pulp stock is prevented by a device which discharges a gaseous fluid into the space lying in front of this surface.