A delayed diagnosis of insulinoma remains a clinical issue. Hypoglycemic symptoms can mimic neuropsychiatric disorders such as epilepsy. A 27-year-old woman with a history of epilepsy and anti-epileptic drugs (AEDs) developed repeated seizures and neuropsychiatric symptoms after a 9-year asymptomatic interval. She had received transient treatment with AEDs before the possibility of hypoglycemia was considered. Following a clinical diagnosis of insulinoma, distal pancreatectomy was performed; her seizures didn't occur again. The early diagnosis of insulinoma requires vigilance not only for hypoglycemia in patients with neuropsychiatric symptoms but also for the possible masking effects of a history of epilepsy and preceding AED usage.