A tubular furnace as a reactor for producing carbon black is disclosed having a constriction in at least the area of the spray device for the carbon black starting material which is oriented along the reactor axis. A combustion chamber for generating the gaseous pyrolysis medium for the carbon black raw material is laterally located in front of the constriction. The central axes of the reactor tube and the combustion chamber are in the same plane and are essentially at right angles to one another. In addition, a method for producing furnace carbon blacks is disclosed in which the described reactor can be used.