To provide a method developed from an idea of using woody fuel instead of fossil fuel since granular bed soil to be used in a nursery process of farming is produced by using a dry kiln device in areas mainly occupied by agriculture and forestry, and business is hard to manage due to increasing prices of gas and kerosene used as fuel, but fuel such as woody chips exist in large amounts in mountain areas.

By using an air superheating supply method in a heat generating method using spiral combustion (centrifugal eddy current system, abundant ventilation system), further high temperature energy can be generated. Accordingly, utilization in a heating method field using wood as fuel can be enlarged.