Abstract: The present invention provides a reagent and a method of safely, conveniently, and specifically diagnosing Crohn's disease. Provided are a diagnostic method for Crohn's disease in a subject, including measuring antibodies against one kind or more of dietary components selected from the group consisting of grapefruit, alfalfa, avocado, cabbage, green pepper, lettuce, onion, potato (white), spinach, tomato, oat, pecan, yeast, cane sugar, celery, buckwheat, corn, rice and soy, and a diagnostic reagent or kit for Crohn's disease, containing the above-mentioned preparation of a dietary component.