A two-dimensional image sensor such as a stimulable phosphor sheet is exposed under vacuum to an electron beam that has passed through a specimen to cause the two-dimensional image sensor to store the energy of the electron beam, then stimulating energy is applied to the two-dimensional image sensor for discharging light therefrom which represents the stored energy of the electron beam, and the light discharged from the two-dimensional image sensor is photoelectrically detected to produce an image signal. A reference image signal is also produced in a similar manner by exposing the two-dimensional image sensor to an electron beam that has not passed through the specimen. One of the first-mentioned image signal and the reference image signal is subtracted from the other per pixel to generate a difference signal, from which an electron microscope image of the specimen is reproduced.