In order to observe the dry line formed on the surface of wood fibre pulp on the moving wire in a Fourdrinier paper machine, its--atrea of appearance is scanned at a low angle with a thin beam cluster (A1, B1) which, being generated by a laser radiation source (20) and representing one or more discrete wavelengths, is distinguishable from the background light. Hereby the surface following the dry line (I) reflects diffusively a major part and specularly a minor part of the arriving radiation than the surface preceding it. A detector (30) located above the level of the wire observes the intensity of the diffusely reflected radiation (A3, B3) hitting it and delivers continuously, as the beam cluster scans the pulp surface, a signal which is proportional to said intensity. The signal is digitized repeatedly and processed in real time in a computer, whereby a series of signals in the direction of the paper machine produces one dry line point and, if repeated for different positions in the cross direction, the dry line consisting of such points. The dry line and the values of quantities and deviations characterizing it are displayed by graphical and numerical terminals and actuators of the paper machine are controlled according to it automatically, synchronously with the scanning of the dry line region. The method can be implemented also in such paper machine environments, to which the earlier methods for structural reasons do not apply.