In ginseng saponin, $ginsenoside-Rb_1$ was contained the most abundantly. But ginsenoside-Rd which is similar to ginsenoside $Rb_1$ in structure was known to be superior to $ginsenoside-Rb_1$ pharmaceutically. A strain of Rhizopus japonicus is able to produce the convertible enzyme which can convert selectively $ginsenoside-Rb_1$ to ginsenoside-Rd without the change of any other ginsenoside. The strain can produce the most enzyme after 5 day-culture on wheat bran medium. The enzyme production was promoted best efficiently by addition of red ginseng powder in ginseng products, xylose in sugars, laminarin in polysaccharides, naringin in flavonoids, and potassium nitrate in nitrogen substrates.