Infants with an ileostomy can be at high risk of hypoglycemia because of inadequate nutritional intake; however, there are no reports investigating blood glucose (BG) in infants with ileostomy. We experienced a case of an extremely low birth weight infant who was born at 24 wk of gestation and weighted 623 g. He received an ileostomy because of an intestinal perforation. After the ileostomy, he had recurrent hypoglycemia. Continuous glucose monitoring showed fluctuation of BG levels (postprandial BG elevations and subsequent declines) and non-fasting hypoglycemia, which were undetectable with intermittent fasting BG measurement. The fluctuation of BG levels and non-fasting hypoglycemia improved after closure of the ileostomy. Patients with ileostomy may present with hypoglycemia that is undetectable with intermittent fasting BG measurement. In this case, continuous glucose monitoring was very useful for detecting fluctuation of BG levels and hypoglycemic episodes. Therefore, we recommend that continuous glucose monitoring be performed in infants with an ileostomy to confirm whether they have hypoglycemia or a fluctuation in BG levels. Further studies on the postprandial dynamics of various hormones in infants with ileostomy are required.