The present invention relates more particularly to the structure of the electrodes arranged in the ionization chambers of a device for monitoring the position, intensity, uniformity and directivity of an ionization chamber belonging to a device for monitoring ionizing beam constituted by charged or unchanged particles. This monitoring device comprises identical electrodes designed to be disposed in parallel with each other and one below the other, each comprising 2n lamellar elements electrically conductive and electrically insulated from each other by 2n - 1 insulating, parallel bands, n being an integer greater than 1. The 2n lamellar elements of an electrode being disposed symmetrically to either side of an axis splitting one electrode into two equal parts, the insulating bands of the electrode making an angle ? with those of the other electrode. Simplicity of manufacture and utilisation are two of the advantages of these electrodes which can incorporate lamellar elements of the same area.