We herein report the first case of an older woman with type 1 diabetes who attempted suicide by subcutaneously injecting herself with an overdose of insulin degludec and lispro. A 70-year-old woman being treated for type 1 diabetes and depression was admitted to our hospital three hours after attempting suicide via the injection of 300 U of degludec and 300 U of lispro. On admission, she was drowsy, and her capillary blood glucose level was less than 30 mg/dl, while her serum insulin level was remarkably elevated at 2972.1 μ U/ml. We closely monitored her serum insulin and blood glucose levels and administered intravenous glucose as needed, and she subsequently recovered from hypoglycemia approximately 30 hours after the insulin overdose. However, because her serum insulin level was still 1,327.0 μ U/ml at 36 hours after the overdose, intravenous glucose injections were continued for five days to prevent hypoglycemia. This is the first case of prolonged hypoglycemia caused by an insulin degludec overdose in a patient with type 1 diabetes. Monitoring the daily plasma insulin level was useful for predicting and preventing severe hypoglycemic episodes in our patient. The present case highlights the need for a longer period of treatment with intravascular glucose infusion in patients with an overdose of insulin degludec.