A culture medium composition for examining the presence or absence of dental caries pathogen in the mouth of a subject (adult, child, infant or baby) as one means for preventing dental caries, which comprises in combination of (a) a basal culture medium having a good storage stability, which is used for culturing collected saliva (0.1 mi) of the subject and determining the presence or absence of the microorganism and/or the number of cells and (b) a dry support containing various test reagents (in the dry state) to be added to the basal culture medium, optionally with (c) a vessel containing the basal culture medium (a) and the support (b) therein, which functions also as a vessel for culturing the microorganism.