Aspergillus oryzae has a strong phosphatase activity. FAD is hydrolyzed first to FMN with the enzyme and then quantitatively to riboflavin and inorganic phosphate. The latter reaction would be carried out with phosphomonoesterase II, this inference being different from the Warburg theory. The properties of the enzyme was investigated employing Takadiastase directly. Reaction velocity is linear to time at least during a certain period from the beginning and Michaelis-Menten constant Km is 1.28×10^<13> mol. Activity of the enzyme shows apparent decrease at pH 3.2 and 6.4. F', Ag' and molybdates inhibit the enzyme action and Mg" has neither inhibiting nor activating effect. The inhibition observed with Ag ion seems to due to the formation of riboflavin-AgNO_3 complex. Competitive inhibition with β-glycerophosphate was not observed in the concentration less than 10^<-2> moles per litre. Riboflavin-AgNO_3 complex has an absorption maximum at 267 mμ and riboflavin at 265 mμ.