PURPOSE: To prepare the subject fermented milk free from the excessive rise of acidity at high temperature, having high safety and useful as yogurt, etc., by carrying out mixed inoculation of specific plural revertants to milk or processed milk and culturing the strains.

CONSTITUTION: The objective fermented milk is prepared by using (A) a revertant of Lactobacillus helveticus (e.g. FERM 10635) free from lactobacillus fermentation property but easily developing a lactose-fermentative revertant and (B) a revertant of Streptococcus thermophilus (e.g. FERM 10636) having weak lactose-fermenting activity and developing a revertant having strong lactose-fermenting activity, carrying out the mixed inoculation of both revertants to a milk or processed milk mixed with glucose, fructose, sucrose or a material containing the sugars in an amount less than the amount giving the target acidity and culturing the strains at 30-45°C.