With the growing interest in the quality of emergency medical services, much effort is being made to minimize medical errors. Smart emergency room, where ICT technology is actively applied to emergency room operation, is accelerating. In this study, a medical error index reflecting the real-time medical error situation is developed, and an emergency room operation rule based on this index is proposed. The medical error index is calculated by reflecting the patient/communication factors, medical task factors, and medical staff/environment factors. To verify the effectiveness of the proposed operating rule, a simulation experiment was conducted for a virtual small emergency room. Simulation results show excellent performance in terms of error count and error severity.