A measuring unit 13 is provided inside a rotary drum 11. The measuring unit 13 is provided with a light transmitting member 13a and an optical sensor 13b that are provided at the central region on the inner wall of the other end portion 11b. When particles are subjected to a coating treatment, information as to the properties (such as the coating thickness, the moisture, the coating performance, and the impurity) of the particles of a particle layer A being brought into contact with the surface of the light transmitting member 13a is measured on a real-time basis by the NIR sensor 13b via the light transmitting member 13a. The data is processed by the processing unit of the NRI spectroscopic analyzer to thereby perform monitoring. By appropriately adjusting the coating operation conditions (such as the supplied air volume, the supplied air temperature, the spraying condition, and the rotation speed of the rotary drum 1) by feedback control or a manual operation according to the result, it becomes possible to perform the coating treatment of high quality.
