An aggregation pattern detecting apparatus includes a microplate having a plurality of reactive vessels in which aggregation patterns are formed, a light source arranged above the microplate, and a light sensor arrangement positioned below the microplate. The microplate is removably fastened to a main body, and a movable frame is movably supported on the main body. The light source and light sensor arrangement are carried on the movable frame for movement therewith. A driving mechanism effects movement of the movable frame and is controlled by a main control section. A reference plate is provided separately from the microplate, and has a plurality of through holes therein which correspond to the reactive vessels of the microplate. The reference plate is removably fastenable to the main body at the normal operative position of the microplate. With the reference plate in this position, the distance from a starting point of the driving mechanism to the through holes of the reference plate is measured and stored in the main control section. Based on these stored distances, the main control section determines appropriate operational positions for the light sensor.