A 5 year-old boy experienced anaphylaxis after eating a jelly product for diet supplement containing erythritol as a major component. Prick test with the jelly product was negative, but the second oral ingestion of the jelly product at home caused another allergic reaction. Prick test with erythritol was negative even at 300 mg/ml, which was almost the solubility limit. Intradermal test was marginally positive at 0.1 mg/ml, and clearly positive at 1 mg/ml or higher concentration. We found subtle dose-response reaction utilizing basophil activation test, examined with 24 hour incubation at the concentration of 40-4000 μg/ml. At the oral challenge test in the hospital, 3 g of erythritol induced remarkable coughing, urticaria, edema, wheezing and hypoxemia. Erythritol is a natural sugar alcohol, with the molecular weight of 122.12, which is recently being widely used for diet supplements, beverages, or drug medicines due to its properties of calorie-free and good-tasting, with easy-to-use physical characteristics. We now have to recognize erythritol as a candidate for food allergen, and to be careful about negative result of prick test.