In situ investigation of displaced chain walls in 80 at% Ni Permalloy under applied in-plane magnetic field has been carried out by out-offocus methods of Lorentz electron microscopy. The magnetic flux in the plane of the film is schematically illustrated. The in-plane magnetic field applied to the foil is produced by a homemade magnetizing stage. By adjusting magnitude and direction of the electric current through the coils of the magnetizing stage, the magnetization reversal process of the magnetic domain wall has been in situ observed.