PURPOSE: To provide a photoelectric tube free from ion spot or ion noise and provided with desired spectral sensitivity characteristic, by practicing the process of applying electricity to and heating a fine wire made of tantalum and titanium at the proper point in the activation stage and forming the surface layer once again.

CONSTITUTION: A tube connected to an evacuating apparatus is subjected to degassing process by baking or the like while being evacuated, and then, the photoelectric cathode is formed by evaporating silver or antimony and alkali metal within the tube. The evaporation of the substances to form the photoelectric cathode is carried out while the tube is kept at the temperature of 150W250°C, for example, and after the evaporation has been finished, a fine wire formed of tantalum and titanium is supplied with electricity to be heated within the tube while the tube is kept at the above temperature. After the heating of the fine wire has been finished, a small amount of cesium and antimony or silver are evaporated within the tube once again monitoring the change in the sensitivity to long-wave light to restore the surface layer once deteriorated. The evaporation is ended when the sensitivity to the long-wave light has reached the maximum value, and then, the tube is slowly cooled and the formation of the photoelectric cathode is thus completed.