To determine the exposure time for taking a picture of an image with an electron microscope without using a mechanism for measuring the brightness of an observed image on a fluorescent screen or the like.

A transmitted image of a sample 4 is taken by a TV camera 7 and displayed on a monitor 11. And a brightness contrast signal from a signal processing circuit 9 and an average brightness signal per frame from an output circuit are input to a microscope control circuit 16. When the transmitted image of a sample 4 is filmed, a film 13 is set in a region of an electron beam after transmitting the sample 4 in a film camera 12. The microscope control circuit 16 determines the proper exposure time based on the input brightness contrast signal and the average brightness signal per frame, and outputs the determined exposure time to a shutter driving circuit 14. Then, a shutter is controlled by the shutter control circuit 14 based on the input exposure time for the transmitted image of the sample 4 being recorded on the film in a proper density.