An automatic tracking apparatus to track a reflector comprising: a surveying machine body which is rotated in the horizontal direction and also is rotated in the vertical direction; an illumination portion disposed in said surveying machine body for illuminating a measurement light toward a reflector; a light receiving portion which is disposed in said surveying machine body and which has an image sensor for receiving a reflection light image of the measurement light illuminated toward said reflector; arithmetic means for calculating a position of the reflection light image in a rectangular area located on said image sensor; a rotation mechanism for rotating said surveying machine body so as to position the reflection light image on a light receiving optical axis of said light receiving portion based on the position obtained by said arithmetic means; and a distance measuring portion disposed in said surveying machine body for measuring the distance from the surveying machine body to the reflector, composed of a light projecting system for projecting a distance measuring light to the reflector, wherein the wavelength of said distance measuring light is different from the wavelength of said measurement light, a light receiving system for receiving the distance measuring light reflected from the reflector, and a measuring circuit for calculating the distance from the surveying machine body to the reflector on the basis of the light projected from the light projecting system and the light received by the light receiving system, characterized in that a light receiving area which is a smaller area than the rectangular area, is provided in the rectangular area located on the image sensor and has said light receiving optical axis as a center, and wherein the area of the light receiving area is changed in accordance with the distance from the surveying machine body to the reflector such that, when the distance changes from a shorter to a longer distance, the area of the light receiving area changes from a broader area to a smaller area. a surveying machine body which is rotated in the horizontal direction and also is rotated in the vertical direction; an illumination portion disposed in said surveying machine body for illuminating a measurement light toward a reflector; a light receiving portion which is disposed in said surveying machine body and which has an image sensor for receiving a reflection light image of the measurement light illuminated toward said reflector; arithmetic means for calculating a position of the reflection light image in a rectangular area located on said image sensor; a rotation mechanism for rotating said surveying machine body so as to position the reflection light image on a light receiving optical axis of said light receiving portion based on the position obtained by said arithmetic means; and a distance measuring portion disposed in said surveying machine body for measuring the distance from the surveying machine body to the reflector, composed of a light projecting system for projecting a distance measuring light to the reflector, wherein the wavelength of said distance measuring light is different from the wavelength of said measurement light, a light receiving system for receiving the distance measuring light reflected from the reflector, and a measuring circuit for calculating the distance from the surveying machine body to the reflector on the basis of the light projected from the light projecting system and the light received by the light receiving system, characterized in that a light receiving area which is a smaller area than the rectangular area, is provided in the rectangular area located on the image sensor and has said light receiving optical axis as a center, and wherein the area of the light receiving area is changed in accordance with the distance from the surveying machine body to the reflector such that, when the distance changes from a shorter to a longer distance, the area of the light receiving area changes from a broader area to a smaller area.