We present here a case of inadvertent insulin overdose (500 units) during surgery in a 51-year-old DM (diabetes mellitus) patient, but the patient recovered without complications after a tightly monitored glucose infusion. Five milliliters (500 units) of insulin instead of 5 units were injected inadvertently to a 51-year-old diabetic patient and the event was detected without delay at the post-anesthesia care unit (PACU). Although the patient was alert and extubated, we cautiously and intensively monitored the vital signs, plasma glucose level, arterial pH, the electrolytes and the fluid intake and output in the PACU. We administered intravenous glucose for 16 hours to avoid hypoglycemia in accordance with the plasma glucose level. The patient was sent to the intensive care unit (ICU) 8 hours after the event, and the patient was discharged at the next day from the ICU. Insulin should be administered with great care in DM patients undergoing surgery, and glucose infusion is an effective method to maintain the blood glucose level during treatment of hypoglycemia patients.