There are provided a novel microorganism Enterobacter sp. B20 strain which produces (E)-4-(2-isocyanovinyl)phenyl .alpha.-rhamnopyranoside having a favorable effect of inhibiting melanogenesis and .alpha.-L-rhamnopyranoside produced thereby. This novel microorganism is a gram-negative bacillus isolated from soil and shows motility owing to polar flagellum. The .alpha.-L-rhamnopyranoside, which is produced by culturing the microorganism under aerobic conditions at a culture temperature of 10 to 37.degree. C. and at a medium pH of about 5 to 9 usually for about 1 to 7 days, is useful as a highly safe and stable whitening agent.