1. A system for obtaining a secondary electron image and reflected electron images of a specimen, having an auto-focusing operation, said system comprising: a scanning electron microscope including an objective lens that converges an electron beam to the specimen, at least one detector that outputs a signal for forming the secondary electron image, and at least one detector that outputs a signal for forming the reflected electron images; and a computer that evaluates a state of a focus based on a combined image which is generated by adding images of said secondary electron image and said reflected electron images in said auto-focusing operation.