To provide a method for preparing soil and an organic compound (such as agricultural, livestock, forest biomass) contaminated with radioactivity such as radioactive cesium (Cs) to soil with contamination concentration of an allowable reference value or less.

In a process of performing composting processing of soil and an organic compound by fermentation of microbes, vapor to be generated due to heat generation is condensed as dew condensation water by a cooling processor, and extraction processing is performed as condensed water containing radioactivity. As a result, the radioactivity such as Cs in the contaminated soil is separated and reduced. A detection method of stereoscopic temperature and humidity of a dew condensation promotion device part, an efficient operation method of a dew condensation promotion process, and an appropriate control method are further constructed.