Five known kaurane type diterpenoids, 16$alpha$H, 17-isovaleryloxy-ent-kauran-19-oic acid (1), 16$alpha$-hydroxy-17-isovaleryloxy-ent-kauran-19-oic acid (2), paniculoside-IV (3), 16$alpha$-hydroxy-ent-kauran-19-oic acid (4), and ent-kaur-16-en-19-oic acid (5) were isolated from the root of Acanthopanax koreanum by repeated column chromatography and reversed phase preparative HPLC. The structures of these compounds were established from physicochemical and spectral data. Among the isolated compounds 16$alpha$H, 17-isovaleryloxy-ent-kauran-19-oic acid (1) showed potent inhibitory activity ($IC_50$ value, 16.2 $mu$ M) on TNF-$alpha$ secretion from HMC-1, a trypsin-stimulated human leukemic mast cell line.