The electron multiplying device according to this invention comprises an electron multiplying unit including dynodes arranged in a plurality of stages. The electron multiplying unit has an incidence opening for an energy beam to be multiplied to enter through, and has the proximal end secured to a base. There is provided a casing for housing the electron multiplying unit. The forward edge of the casing is secured to the base, and a space defined by the base and the casing houses the electron multiplying unit. The casing has an entrance window formed at a position opposed to the incidence opening. Energy beams enter the electron multiplying unit through the entrance window, but the electron multiplying unit itself is housed in the casing to be protected from surrounding air flow and unnecessary energy beams not to be measured.