To provide a feeding method of a material to be treated in a reactor for an organic substance and the like by means of a supercritical fluid or a subcritical fluid that is capable of dispersing a constant amount of the material to be treated into fluid vapor and that is capable of introducing the material to be treated without biting a sheet part and the like in a reaction function.

The reactor is comprised of a cylinder 2 and a compression plunger 4, and is provided with the reaction system for compressing the fluid vapor accompanied by the material to be treated which is introduced into the cylinder 2 by operating the compression plunger 4, reducing the temperature and pressure of the fluid vapor by operating the compression plunger 4 to the opposite direction after the completion of the chemical reaction of the material to be treated by means of the compression, taking out the fluid including the obtained product from the cylinder 2 and blowing new fluid vapor into the cylinder 2 to perform a periodic intake/exhaust stroke. The material to be treated is carried on the high pressure fluid spray, and is dispersed and introduced into an intake port 3a during a period of inhaling the liquid vapor.