To provide a method, in which damage by a noxious insect such as a termite or mold or the like of an underfloor wooden structure section is prevented effectively.

A dry-distillation gas generator 1 drily distilling scrap wood or unused wood 2 such as lumber by thinning is loaded singly, others are all sealed up while leaving two ventilating openings 8 in the underfloor section of a building, a dry-distillation gas 5 is introduced from one, the underfloor section is filled with the dry-distillation gas 5, the state is kept for 4-5 hr, and soil surfaces and a wooden structure section in the underfloor section are fermigated. Insecticide and sterilization are conducted by the dry-distillation gas and wooden vinegar contained in the gas while charcoal manufactured by dry-distillation can be utilized for crying the underfloor section at the time.