It takes management method at harvest biomass for the present invention, by measuring the amount of fatty acids in plants, and obtains the percentage of linolenic acid to total fatty acid content, based on the percentage of the acquired linolenic acid, at harvest of plants It includes the estimation process for estimating the amount of biomass.本発明にかかる収穫時バイオマス量の管理方法は、植物中の脂肪酸の含有量を測定し、全脂肪酸量に対するリノレン酸の割合を取得し、取得したリノレン酸の割合に基づいて、植物の収穫時バイオマス量を推定する推定工程を含む。