Queuing theory consists of two probability distributions. One is the arrival distribution and the other is the service distribution at the queue. A model should be described with Kendall's notation to solve problems in the model which is set to reduce waiting time of outpatients. In this paper, the queuing model is, firstly, searched by queuing theory for an outpatient department. Secondly, an assignment rule is proposed by queuing simulations to reduce expected waiting time in waiting queues of outpatients. The rule is decided by four situations on the vacancy in outpatient department etc., and the sequence in the waiting lines. Finally, effectiveness of the proposed rule is verified through numerical experiments.