PURPOSE: To obtain a photo-cathode with a photoelectric conversion factor high over a wide range of wavelengths by forming an n-type semiconductor thin film of prescribed compositions on a p-type amorphous Si group thin film so as to form hetero-coupling.

CONSTITUTION: An n-type semiconductor thin film 3 is formed to form hetero- coupling on a p-type amorphous Si group photoconductive thin film 2 made of an amorphous Si alloy which has an energy gap appropriate for incident power energy. Substances used in the n-type semiconductor thin film are as follows: Cs2O small in its electron affinity or work function, or Oxides of Ba, Sr, Ca, B, La, large in their secondary electron emission factors, or compounds such as LaBa, BaCO3, SrCO3, CaCO3, BaCO3.SrCO3.CaCO3, BaO.SrO.CaO. A photo-cathode with a photoconductive conversion factor high over a wide range of wavelengths and without generating pollution can be formed at small cost.