An electron spectrometer comprising: an energy analyzer section that energy-analyzes electrons emitted from a specimen; a micro-channel plate that amplifies the electrons analyzed by the energy analyzer section; a fluorescent screen that converts the electrons amplified by the micro-channel plate into light; and a camera that photographs the fluorescent screen; characterised in that the electron spectrometer further comprises: an effective range calculation section that calculates an effective range of the fluorescent screen within a camera image photographed by the camera, the effective range calculation section performing a process that acquires a plurality of the camera images photographed while causing the energy analyzer section to analyze the electrons with a different center energy, a process that converts the plurality of camera images respectively into a plurality of spectra, and a process that calculates the effective range of the fluorescent screen within the camera image based on the plurality of spectra, the effective range calculation section, in the process of acquiring the plurality of the camera images, acquiring the plurality of the camera images by continuously photographing the photoelectron image with the camera while changing the center energy of the energy analyzer section so that a peak position of each of the plurality of spectra moves across the effective range of the fluorescent screen, and the effective range calculation section, in the process of calculating the effective range of the fluorescent screen, determining a maximum brightness position within the camera image from each of the plurality of spectra, drawing a graph in which the center energy and the maximum brightness position are respectively plotted along a first axis and a second axis with respect to each of the plurality of spectra, and calculating the effective range of the fluorescent screen from the graph. an energy analyzer section that energy-analyzes electrons emitted from a specimen; a micro-channel plate that amplifies the electrons analyzed by the energy analyzer section; a fluorescent screen that converts the electrons amplified by the micro-channel plate into light; and a camera that photographs the fluorescent screen; characterised in that the electron spectrometer further comprises: an effective range calculation section that calculates an effective range of the fluorescent screen within a camera image photographed by the camera, the effective range calculation section performing a process that acquires a plurality of the camera images photographed while causing the energy analyzer section to analyze the electrons with a different center energy, a process that converts the plurality of camera images respectively into a plurality of spectra, and a process that calculates the effective range of the fluorescent screen within the camera image based on the plurality of spectra, the effective range calculation section, in the process of acquiring the plurality of the camera images, acquiring the plurality of the camera images by continuously photographing the photoelectron image with the camera while changing the center energy of the energy analyzer section so that a peak position of each of the plurality of spectra moves across the effective range of the fluorescent screen, and the effective range calculation section, in the process of calculating the effective range of the fluorescent screen, determining a maximum brightness position within the camera image from each of the plurality of spectra, drawing a graph in which the center energy and the maximum brightness position are respectively plotted along a first axis and a second axis with respect to each of the plurality of spectra, and calculating the effective range of the fluorescent screen from the graph. an effective range calculation section that calculates an effective range of the fluorescent screen within a camera image photographed by the camera, the effective range calculation section performing a process that acquires a plurality of the camera images photographed while causing the energy analyzer section to analyze the electrons with a different center energy, a process that converts the plurality of camera images respectively into a plurality of spectra, and a process that calculates the effective range of the fluorescent screen within the camera image based on the plurality of spectra, the effective range calculation section, in the process of acquiring the plurality of the camera images, acquiring the plurality of the camera images by continuously photographing the photoelectron image with the camera while changing the center energy of the energy analyzer section so that a peak position of each of the plurality of spectra moves across the effective range of the fluorescent screen, and the effective range calculation section, in the process of calculating the effective range of the fluorescent screen, determining a maximum brightness position within the camera image from each of the plurality of spectra, drawing a graph in which the center energy and the maximum brightness position are respectively plotted along a first axis and a second axis with respect to each of the plurality of spectra, and calculating the effective range of the fluorescent screen from the graph.