We report the case of a 5-month-old female infant who had allergic enterocolitis with protein-losing enteropathy and had low birth weight. Until the age of 4 months, she was fed only breast milk and did not experience any related problems. When she was 5-months-old, she was admitted to our hospital with the chief complaints of vomiting and diarrhea. She had whole body edema, thoracic and abdominal ascites, and bloody stools. Laboratory examinations showed the presence of hypoalbuminemia, hypogammaglobulinemia, and an elevated CRP level. Subsequently, she was fed an elemental diet with enteral tube feeding, and her condition gradually improved. However, 2 weeks later, her symptoms reappeared, and therefore, she was admitted to the National Center for Child Health and Development. She underwent upper and colon endoscopies and was found out to have multiple inflammatory lesions in the area extending from the cecum to the rectum. The biopsy findings showed features of eosinophilic enterocolitis and she was diagnosed with allergic eosinophilic enterocolitis. Subsequently, an elemental formula that does not include soybean was used for feeding the infant. The symptoms resolved and her condition improved; currently, she is well and is showing normal development. We have reported this case because allergic enterocolitis is becoming a topic of concern.