A milk raw material is fermented by using Lactococcus lactis not having cell wall-enveloped proteinase and a Bifidobacterium bacterium. The Lactococcus lactis is, for example, a Lactococcus lactis having a property that, when this bacterium and the Bifidobacterium bacterium are inoculated into a medium containing 1% (W/W) of glucose and 10% (W/W) of reduced skim milk powder in amounts of 5.0 x 106 to 2.0 x 108 CFU and 1.0 x 107 to 3.0 x 109 CFU per 1 ml of the medium, respectively, and cultured, and the medium is rapidly cooled to 10°C from culture temperature when pH of the medium becomes 4.6 to 5.5, and stored at 10°C for 2 weeks, survival rate of the Bifidobacterium bacterium is maintained to be 30% or more.
