1. A method of computing the area and number of articles in an automatic sorting apparatus including an image pickup unit for producing images of the articles, an image memory for storing, as a multiplicity of pixels, the images of the articles one frame at a time, a sorting device for deriving the area of an imaged article from the number of pixels forming the article image and for comparing the area of the imaged article with a preset range of grades to produce selection information indicative of a particular grade to which the article belongs, and means for sorting out the article dependent on the size thereof according to the selection information, said method comprising the steps of: (a) determining the black/white level of each pixel by raster scanning the pixels forming the image of the article and storing the starting and ending positions of a continuous black-level portion in an ith scanning step; (b) determining whether a black-level portion in an (i+1)th scanning step is contiguous to the black-level portion in the ith scanning step by employing the stored starting and ending positions; (c) adding the number of pixels of the black-level portion in the (i+1)th scanning step to the total number of pixels of the black-level portions in scanning steps up to the ith scanning step when the black-level portions in the ith and (i+1)th scanning steps are contiguous to each other, or separately storing the number of pixels of the black-level portion in the (i+1)th scanning step when the black-level portions in the ith and (i+1)th scanning steps are discontinuous from each other; and (d) identifying the number of articles in one frame as being two when black-level portions in all scanning steps following the (i+1)th scanning step are discontinuous from the black-level portions in the ith and (i+1)th scanning steps, or adding the number of pixels of the black-level in a current scanning line to the total number of pixels in the ith and (i+1)th scanning steps when black-level portions in scanning steps following the (i+1)th scanning step and up to the current scanning line are contiguous to the black-level portions in the ith and (i+l)th scanning steps and identifying the number of articles in one frame as one, with the total number of pixels representing the area of the one article.