Abstract: A secondary ion mass analyzing apparatus is suitable for a depth directional analysis of a specimen. The apparatus includes means for forming an image of said secondary ions, an aperture disposed on a position in which the secondary ion image is formed, means for detecting the secondary ions which have passed through the aperture and for converting the detected ions into electrical signals, and means for displaying an image of said aperture based on the electrical signals. In such a manner, the aperture is disposed on the secondary ion image forming position. The image of the aperture is displayed on the image displaying apparatus by using the secondary ions which have passed through the aperture. If the ion image is not formed on the position of the aperture, the contour of the aperture image would be unclear while if the ion image is formed on the position of the aperture, the contour of the aperture image would be clear.