|Social solidarity, equity in financing, and efficiency in administration have been core issues in the development of Korean health insurance reformation since 1988. This study is to investigate the trend of administrative cost in Korean National Health Insurance from various aspects. For the analysis of administrative cost, the expenditures of each insurance society and the National Health Insurance Corporation are divided into 4 items of (1) insurance benefit, (2) administrative cost, (3) an agency provision accounts, (4) other expenses, and then they are reorganized. The analyses based on 5 types of the health insurance administrative cost showed that efficiency in administration has been improved generally. We, however, should consider qualitative aspects such as customer`s satisfaction with health insurance administration, prompt service, control of unjust expenditure (unjust claims), and provision of medical service including health consultation in assessing efficiency of administration. And, in order to connect the administrative costs of health insurance with efficiency, we need to give a fundamentally new definition, which can contain elaborateness of expenditure in details including the structure and evaluation method of administrative costs. It may be necessary to develop new indicators or analyzing methods hereafter.