Abstract: A wood preservative additive composition which is effective against wood rot fungi (brown rot fungi, white rot fungi, soil bacteria and soft rot fungi), does not require specific treatment as defined in Water Pollution Control Law or the like, and is environmentally friendly when treating wastes. The composition has wood preservatives (Cyproconazole or the like) which are effective against brown rot fungi and/or white rot fungi, and wood preservatives (p-cumylphenol or the like) which are effective against soil bacteria and/or soft rot fungi.