An electron tube (1) in which one end of an insulating tube (9) projects toward the inside part of an enclosure (2), and an avalanche photodiode (APD) (15) is provided to the one end of the insulating tube (9). The other end of the insulating tube (9) is connected to an outer stem (6) of the enclosure (2). An alkali source (27) is disposed inside the enclosure (2) so as to produce alkaline metal vapor and to form a photoelectric surface (11) in a predetermined area of the inner wall of the enclosure (2). The alkali source (27) is separated from the insulating tube (9) by separating members (21', 23', 26). The alkaline metal vapor produced when the electron tube (1) is fabricated does not deposit on the insulating tube (9) because of the presence of the separating members (21', 23', 26). The withstanding voltage between the enclosure (2) and the APD (15) does not lower, and deterioration of the efficiency of entrance of electrons into the APD (15) because of the adverse effect on the electric field inside the electron tube (1) in prevented.