Abstract: A scanning electron microscope capable of securing a high ion detection efficiency in spite of a short working distance and the adaptability for high resolution is disclosed. An ion detector for detecting ions is arranged nearer to an electron source than a first pressure limiting aperture for maintaining the specimen chamber in a predetermined vacuum. This configuration makes it possible to achieve the high resolution of the semi-in-lens objective lens on one hand and to detect the ions generated by the collision between the secondary electrons and the gas molecules on the other hand.