[Object] An object of the present invention is to provide a method for manufacturing fermented milk that can keep good taste and quality regardless of the condition of lactobacillus and enzymes.[Means for Solving] The invention is directed to a method for manufacturing fermented milk that comprises a step of fermenting the ingredient milk that comprises enzyme and a step of executing de-oxygen treatment before the fermenting step. The enzyme has best pH of activation in neutral region and loses its activity in acid region and can decompose lactose which is included in the ingredient milk when the enzyme is in an active state. As shown by the working Examples herein, it is possible to keep good taste and quality regardless of the condition of lactobacillus and enzymes by executing de-oxygen treatment to the ingredient milk before fermentation step.[Selected Figure] FIG. 3