The purpose of this study was to introduce the methodology of intervention analysis with time series data and to investigate the influence of the patient referral system on medical care utilization in Kangwha county. The data were obtained at the Kangwha Medical Insurance Society and we analysed the material based on the outpatient care fee. The results were as follows: 1. The average outpatient care utilization in the hospital decreased by 41.7% due to the patient referral system. 2. The utilization of the health institution increased by 278.8 persons per month due to the patient referral system. 3. The patient referral system did not influence the total outpatient care utilization. The methodology of intervention analysis, which detected the effect of intervention, will be helpful to the study of public health area.