A purple non-sulfur bacteria useful in preventing and/or ameliorating at least one disease selected from the group consisting of inflammatory diseases, allergic diseases, and autoimmune diseases and having high safety is provided. A preventive and/or ameliorative agent is for at least one disease selected from the group consisting of inflammatory diseases, allergic diseases, and autoimmune diseases and includes at least one of a purple non-sulfur bacteria and a culture material obtained by culturing the purple non-sulfur bacteria. The purple non-sulfur bacteria includes Rhodobacter azotoformans.
