PURPOSE: To produce an acidic fermented milk serum product having a well- mellowed and rich flavor by preventing an uncomfortable smell from generating accompanying yeast fermentation of milk serum containing a high nitrogen source.

CONSTITUTION: This acidic fermented milk serum product is produced by carrying out Lactobacillus fermentation of milk serum and/or adding an acid and a sugar until the sugar concentration reaches 20 to 35wt.% thereto, further adding a tannin thereto, adjusting pH to 2.8 to 4.5 and carrying out yeast fermentation of the resultant solution. As the tannin, tea-derived tannin or fruit-derived tannin or their combination can be used.