PURPOSE: To improve yield and to save labor by making particle diameters of granulated compound fertilizer close to target particle diameters by intermittent photographed image of the granulated compound fertilizer and carrying out specific control in a stable state and in unstable state.

CONSTITUTION: A raw material 1 is mixed with a returned raw material 2 by a blender 3 and thrown into a granulator 10. In the granulator 10, the blended raw materials are mixed with granulating water 4, sulfuric acid 5, ammonia 6 and phosphoric acid 7 rolled and granulated. The amounts of the granulating water 4, the sulfuric acid 5, the ammonia 6 and the phosphoric acid 7 indicated in product specifications are fed in the vicinity of the granulator 10. The production of compound fertilizer is controlled by using a collected table called particle size diagnosing S/H matrix 100 and integrating the table into a computer. A skilled specialist of granulation grasps the state of particles sent from the granulator 10 by observation, sieve and touch, reasons by using various knowledge of granulation and rules by experience through the granulated state during the operation and experience for many years and solves problems.