An improved reactor for a high-temperature deposition reaction on seed particles is constructed with a fluidized bed which is divided into a heating zone and a reaction zone by a partition. Seed particles in the heating zone are fluidized by a carrier gas and are heated by microwaves. On the other hand, the reaction zone for the deposition reaction, through which reaction gases pass, is heated by particle mixing between the reaction zone and the upper section of the heating zone. Subsequently, a desired reaction temperature at the reaction zone is maintained stable without deteriorating the microwave heating of the heating zone.