We recently encountered a 96-year-old Japanese woman who suffered from frequent hypoglycemia. Endocrinological and imaging data eliminated the possibility of insulinoma, whereas oral glucose tolerance testing revealed impaired glucose tolerance and subsequent reactive hypoglycemia. The patterns between insulin or C-peptide secretions and glucose excursions demonstrated that the discrepancy occurred in the late postprandial stage. Administration of small doses of alpha-glucosidase inhibitor (alpha-GI) dramatically inhibited the rapid rise and subsequent precipitous fall of plasma glucose. Reactive hypoglycemia may be one of the important cause of hypoglycemia in the elderly, and alpha-GI could effectively and safely prevent such hypoglycemic attacks in those patients.