allowing (i) a culture or treated culture of a microorganism having N-acetylneuraminic acid synthetase activity, (ii) a culture or treated culture of a microorganism belonging to the genus Escherichia or Corynebacterium and capable of producing phosphoenolpyruvic acid, (iii) N-acetylmannosamine, and (iv) glucose or fructose to be present in an aqueous medium to form and accumulate N-acetylneuraminic acid in aqueous medium, wherein said N-acetylmannosamine is produced in said aqueous medium or added thereto, and said glucose or fructose is added to said aqueous medium, and wherein free phosphoenolpyruvic acid is not added to said aqueous medium; and

recovering N-acetylneuraminic acid from the aqueous medium,

wherein said treated cultures are independently selected from the group consisting of concentrated culture, dried culture, cells obtained by centrifuging the culture, dried cells, freeze-dried cells, immobilized cells, enzyme-treated cells, surfactant-treated cells and solvent-treated cells, wherein said treated culture continues to have the same enzyme activity as the culture.