To provide a microorganism and an ingredient thereof that contribute to prevention and treatment of immune diseases including allergy, autoimmune diseases and inflammatory bowel diseases (e.g., large-intestinal ulcer), a method of effectively selecting the microorganism, and a method of efficiently inducing immunoregulatory cells that play an important role on maintaining immunological homeostasis using the microorganism or the ingredient thereof. The present invention provides a Lactococcus and an ingredient thereof which induce production of IL-10 from mammalian dendritic or spleen cells, a method of obtaining the microorganism or the ingredient thereof by co-culturing a microorganism such as Lactococcus strains with mammalian dendritic or spleen cells to select a microbial cell having a high IL-10 production ability, a method of selecting the microorganism or the ingredient thereof by co-culturing an intestinal epithelial cell with a microorganism such as the lactic acid bacteria and selecting the cells on the basis of a caspase-1 activity and the ability of inducing the production of IL-18, and a food product or material and an animal feeding stuff or medical and pharmaceutical products which contain the lactic acid bacteria as an effective ingredient.