PROBLEM TO BE SOLVED: To provide a lumber mildewproof composition effective against a wide range of mold on which sufficient effects are so far unattainable by a combination of conventional lumber mildewproof agents.SOLUTION: A lumber mildewproof composition contains at least one selected from tebuconazole, propiconazole, 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one and 2-(4-thiazolyl)benzimidazole as an effective ingredient.