[Problem]A reaction disk capable of compact and inexpensive design of an automatic analyzer is provided so that the constituents in suspension such as blood sampled from a patient at a sick room or other place in a hospital can be analyzed immediately and quantitatively. [Means of Solution]A separation cell and a determination cell are formed in a same reaction disk so as to keep their upright position even during rotation of the both cells, the separation cell being formed to prevent poured suspension therein from flowing out during centrifugal separation, and a supernatant separated by centrifugation from the suspension contained in the separation cell being dispensed to the determination cell to analyze a target substance in the supernatant. [Selected Drawing] FIG. 3