PURPOSE: To make an accurate decision without accurate positioning by forming an image of the bottom surface of a reaction container on a two-dimensional photodetecting element array, calculating the contour of the image form their outputs, and deciding on a pattern on the basis of output of photodetecting elements within the contour.

CONSTITUTION: The particle flocculation pattern formed on the bottom surface of the reaction container 1 when particles in the reaction solution in the reaction container 1 settle is detected photoelectrically to make a decision. For the purpose, a microplate 2 made of a transparent member having containers 1 in a matrix is conveyed intermittently as shown by an arrow, and the bottom surface of the container 1 is lighted by a light emitting part 3 which extends at right angles of said conveyance direction to form an image on the two-dimensional photodetecting element array 5 through a lens 4 and makes a scanning. Further, the photodetection area of the element array 5 is made larger than the image of the bottom surface, and the position of the image is calculated to provide feedback to the movement of the plate 2 so that the whole image of the bottom surface is positioned in the photodetection area. Further, the decision on the flocculation pattern is made on the basis of data at the center part of the bottom surface image and data at the circumferential part.