The present invention relates to a substance or method for preventing or treating mastitis in ruminant animals, particularly in dairy cows. Specifically provided are: a preventive or therapeutic agent, a feed additive, a feed, and a pharmaceutical composition for ruminant animal mastitis, characterized by comprising, as an active ingredient, a Bacillus genus bacterium having the property of being capable of increasing or maintaining CD11c+ CD172a+ blood dendritic cell gate 2 in cows.