The use of automated external defibrillators (AEDs) has increased the number of survivors of out-of-hospital cardiac arrest. The AED has a high specificity and moderately high sensitivity in detecting rhythms appropriately treated by defibrillation. However, a few shockable rhythms are misdiagnosed by the AED. Two cases of inappropriate analyses by AEDs in patients with in-hospital ventricular fibrillation are presented. In the first case, the AED failed to recognize ventricular fibrillation because of the presence of pacemaker spikes. In the second case, the fine ventricular fibrillation and the presence of artifacts were suspected as the causes of inappropriate analysis by the AED. Both patients were resuscitated by advanced cardiovascular life support with a manual defibrillator. Trained healthcare providers should be aware of the limitations of AED in specific situations.