An inexpensive, efficient device that supplies a transmission mode to the conventional SEM has been developed. The transmitted electrons strike a metal plate, and these generate secondary electrons that are proportional to the quantity of the transmitted electrons. The generated electrons are collected by the secondary electron detector. Hence, the performance of this device is influenced by the number of secondary electrons generated in the metal plate. In order to construct a device that can attain the best transmitted electron image, the signal-to-noise ratio of images, obtained from various trial devices, were measured by a newly-developed digital image processing program. When the material and shape of the device are selected, to produce highsecondary emission, the efficiency of the device compares with that of a relatively expensive standard detector system (scintillator detector).