A method and apparatus for discriminating aggregation patterns includes detection of an image of the aggregation pattern and dividing the image into a plurality of image line data using a photoelectric element. The image line data defines an image waveform and includes a maximum image value. High and low image levels are defined based on the maximum image value. These high and low image levels are used to determine first and second areas of the aggregation pattern image, which first and second areas respectively include image values at least as large as the high and low image levels. An area difference between the first and second areas and a level difference between the high and low image levels are then calculated, along with a ratio of the area difference to the level difference. The calculated area difference and ratio are then compared with a predetermined reference value to discriminate whether or not the aggregation pattern is positive.
