We experienced a case of unexpected atrial fibrillation (AF) developed during induction of general anesthesia. The patient was a 71-year-old woman diagnosed with a cerebral aneurysm. She had no history of heart or lung disease. Preoperative laboratory results, chest x-ray, arterial blood gas analysis and vital signs were within normal limits except ST and T changes in a preoperative ECG. Preoperative echocardiography showed left heart enlargement but other findings were nonspecific. AF combined with decreased blood pressure and increased heart rate developed during induction of general anesthesia. Digoxin and esmolol were given for heart rate control. Propafenone was used for chemical cardioversion through a naso-gastric tube. Thirty minutes after medication, AF disappeared and vital signs returned to the preoperative condition.