In the autumn of 1995, a powdery mildew fungus was observed on Angelica keiskei cultivated at Izu Oshima Island, Tokyo, Japan. The leaves, petals of the flower and stems of the plant were covered with amphigenous colonies and persistent mycelium. Conidiophores were straight, 89-159μm in length, and 10-16μm in width. Foot-cells were cylindric, 27-48μm in length, and 8-10μm in width. Conidia were cylindric (-ellipsoid) to oblong, produced singly, and measured (29-)33-46(-52)×13-20μm. The length/width ratio of conidia were between 2.31 to 2.39. Appressoria of conidial germ tubes were lobed, Erysiphe polygoni type proposed by Hirata (1942, 1955). From inoculation experiments of the fungus, we recognized that the fungus on Angelica keiskei was pathogenic to the host plant. Since the cleistothecia of the fungus were not present, we proposed that the fungus was Oidium sp. belonged in E. polygoni type. This is the first report of powdery mildew on cultivated Angelica keiskei.