1. A method for determining an agglutination reaction from a particle pattern formed on an inclined bottom surface of a reaction vessel comprising the steps of: scanning photoelectrically the inclined bottom surface to derive an image signal which represents a two-dimensional image of the particle pattern; processing the image signal into area light intensities by separating the inclined bottom surface into a plurality of areas due to different contours of the inclined bottom surface including decomposing the image signal into a series of concentric rings, each ring representing a different contour area of the image and integrating light intensities in each area to derive the area light intensities; determining an average intensity of each ring; inputting the area light intensities into a neural network to produce output signals; and determining an agglutination reaction based on the output signals.