The saponin mixture has been isolated from the whole body of starfish Acanthaster planci L. (crown of thorns). By use of the glycosidase mixture from the large marine gastropod Charonia lampas, which is known as a natural enemy of A. planci, two genuine sapogenols named thornasterol A (8) (major) and thornasterol B (9) (minor) have been isolated as their diacetates from the saponin mixture. On the basis of chemical reactions and physicochemical evidence, the structures of diacetyl-thornasterol A and diacetyl-thornasterol B have been elucidated to be 3β, 6α-diacetoxy-5α-cholest-9 (11)-en-20-ol-23-one (8a) and 24ε-methyl-3β, 6α-diacetoxy-5α-cholest-9 (11)-en-20-ol-23-one (9a), respectively. Furthermore, the sulfate group contained in the parent saponins of thornasterol A (8) and B (9) has been shown to attach to 3β-OH of the respective sapogenols. It has been also suggested that the pregnane-type sapogenol (1), which has been hitherto isolated not only from A. planci but also from many other species of Asteroidea, may be an artifact sapogenol.