The prevalence of childhood food allergy is increasing, affecting the quality of life of food-allergic patients. Cow milk, a major allergen, accounts for about 20% of all food allergies in Japan. Particularly, it is common in infants, who usually experience the first episode before one year of age. We hereby report a case where the patient instantly developed a contact urticaria after wearing underwear made of milk-containing rayon fiber. The diagnosis was confirmed after carrying out a patch test of the underwear concerned, which showed a positive result for immediate allergic reaction, and an elution test of milk-containing fibers, which revealed casein elutes. Recently, various food proteins have been added to other products, including clothing and cosmetics, which may cause allergic symptoms. Thus, caution is required.