AS A MATERIAL FOR A POROUS LAYER OF A SECONDARY ELECTRON MULTIPLIER USED FOR INSTANCE IN A SECONDARY ELECTRON CONDUCTIVE TYPE CAMERA TUBE OR A SECONDARY ELECTRON MULTIPLIER TUBE, IT IS DESIRABLE TO USE A METAL OXIDE SUCH AS MAGNESIUM OXIDE, ALUMINUM OXIDE OR THE LIKE, WHICH IS CHEMICALLY AND PHYSICALLY STABLE. HOWEVER, IN GENERAL, A METAL OXIDE HAS CONSIDERABLY HIGH EVAPORATION TEMPERATURE SO THAT VACUUM EVAPORATION UNDER LOW VACUUM OF SUCH OXIDE IS DIFFICULT. IN THE METHOD OF THE INVENTION BY UTILIZING AN EUTECTIC PHENOMENON OF SUCH METAL OXIDE WITH SECONDARY ELECTRON EMISSIVE SUBSTANCES HAVING COMPARATIVELY LOW EVAPORATION TEMPERATURE AND BEING A NATURE OF EASINESS OF DEPOSITION UNDER LOW VACUUM, IT IS EASILY ACHIEVED TO EVAPORATE SUCH A METAL OXIDE UNDER LOW VACUUM. THUS A SUPERIOR POROUS LAYER FOR USE FOR INSTANCE IN A SECONDARY ELECTRON MULTIPLIER CAN BE FORMED, WHICH DOES NOT LOSE THE POROUS PROPERTY IN A SUCCEEDING HEAT TREATMENT DURING THE PROCESS OF MANUFACTURING A MULTIPLIER TUBE.