Abstract: A new saponin has been isolated from the aerial parts of polygala japonica Houtt which is used principally as an antiinflammatory drug in, folk-lore medicine. Acidic hydrolysis of this saponin gave a new sapogenin, its structure, C₃₀H₄₈O₅, m. p. 319～321℃, a_D²⁰120°, was elucidated to be 2 a, 3 a, 24-trihydroxyolean-12-ene-28-oic acid mainly by spectroscopic methods and some chemical transformations (Ⅰ).