A camera tube device for use in a television camera comprises an electron gun including a cathode electrode for emission of an electron beam, and first and second grid electrodes respectively having apertures for controlling the diameter of the electron beam. The aperture of the second grid electrode is sufficiently smaller than the aperture of the first grid electrode. The first grid electrode is applied with a positive voltage relative to the cathode electrode and the second grid electrode is applied with a positive voltage relative to the cathode electrode which is higher than that applied to the first grid electrode. The positive voltage applied to the first grid electrode has such a value that forms the electron beam having passed through the aperture of the first grid electrode into a laminar flow beam. As the level of brightness of the object increases, the positive voltage applied to the first grid electrode is decreased to increase the amount of electron beam passing through the aperture of the second grid electrode.
