The wood properties such as the relations between containing water and the strength decline, such as the relations between the deterioration by high-temperature kiln-drying and the strength decline, and such as the relations between the bio-deterioration and the strength decline cause various problems, when a hardwood including sugi (Criptmeria japonica D. Don.) is used as structural members. In this subject, these phenomena are studied, and the establishment of the theory is tried.