An automatic tracking apparatus configured to track a reflector, the apparatus comprising: a surveying machine body; an illumination portion which is disposed in the surveying machine body configured to illuminate the reflector with a measurement light; a light receiving portion which is disposed in the surveying machine body and which has an image sensor configured to receive a reflected light image of the measurement light reflected by said reflector; an arithmetic means configured to calculate a position of the reflected light image from said reflector in a light receiving area of said image sensor based on a received light of said image sensor; a rotation mechanism configured to rotate said surveying machine body in horizontal and vertical directions so as to position the reflected light image on a light receiving optical axis of said light receiving portion based on the position obtained by said arithmetic means; a first storing portion configured to store quantity of light data at each pixel in the light receiving area of said image sensor; and an edge position detecting portion configured to detect a beginning edge position and an end edge position of said reflected light image at each scanning line in said image sensor, wherein said arithmetic means is configured to calculate the quantity of light at each pixel based on the values from said first storing portion and the output of said edge position detecting portion and to calculate the position of the weighted average in horizontal and vertical directions of said reflected light image characterized in that said arithmetic means is configured to read out the quantity of light data at each pixel from said first storing portion, and to obtain a width between said beginning edge position and said end edge position at each scanning line after calling out the positions from said edge position detecting portion, and to decide a scanning line corresponding to the widest line as the position of the weighted average in the vertical direction, and to calculate the position of the weighted average in the horizontal direction of the reflected light image based on the quantity of light of the pixels which exist between said beginning edge position and said end edge position in the scanning line corresponding to the widest line. a surveying machine body; an illumination portion which is disposed in the surveying machine body configured to illuminate the reflector with a measurement light; a light receiving portion which is disposed in the surveying machine body and which has an image sensor configured to receive a reflected light image of the measurement light reflected by said reflector; an arithmetic means configured to calculate a position of the reflected light image from said reflector in a light receiving area of said image sensor based on a received light of said image sensor; a rotation mechanism configured to rotate said surveying machine body in horizontal and vertical directions so as to position the reflected light image on a light receiving optical axis of said light receiving portion based on the position obtained by said arithmetic means; a first storing portion configured to store quantity of light data at each pixel in the light receiving area of said image sensor; and an edge position detecting portion configured to detect a beginning edge position and an end edge position of said reflected light image at each scanning line in said image sensor, wherein said arithmetic means is configured to calculate the quantity of light at each pixel based on the values from said first storing portion and the output of said edge position detecting portion and to calculate the position of the weighted average in horizontal and vertical directions of said reflected light image characterized in that said arithmetic means is configured to read out the quantity of light data at each pixel from said first storing portion, and to obtain a width between said beginning edge position and said end edge position at each scanning line after calling out the positions from said edge position detecting portion, and to decide a scanning line corresponding to the widest line as the position of the weighted average in the vertical direction, and to calculate the position of the weighted average in the horizontal direction of the reflected light image based on the quantity of light of the pixels which exist between said beginning edge position and said end edge position in the scanning line corresponding to the widest line.