An entire emergency department of a general hospital is simulated to examine patient flows. First, times needed for both outpatients and patients arriving via ambulance to be processed in the emergency department are examined. A special-purpose data-generator is designed and developed to create experimental data for executing a simulation. It is found that the patients spend the longer part of their time waiting, depending on the number of patients to be processed. In addition, it is found that the waiting time for available emergency-treatment beds, doctors, drips, and stretchers accounts for the major part of all the waiting time in the emergency department. A stepwise procedure of operations planning is proposed to minimize the patient waiting times, and numerical examples are shown to illustrate the procedure.