Since the introduction of mandatory health insurance in In, the Korean national health insurance(KNHI) has grown rapidly. In 2004, about $96.9\%$ of the total population are covered by the KNHI and the remaining $3.1\%$ by the Medical Aid program. Despite national health insurance system in Korea, private health insurance market has grown rapidly. In 2004, the size of the private health insurance market was estimated at 6,568 billion won. The purpose of this study Is to identify the factors that determine the purchasing decisions of supplementary private health insurance under mandatory national health insurance system in Korea. The data from n04 Busan Health Survey were analysed for the Purpose. The variables in this study are demographic factors, health status and health behavioral factors, health care systemic factors, and socioeconomic factors. For statistical analyse, we used logistic regression. The Findings show that female, economically active age group(especially 35-49 years), persons with better health status or experience of health screening test are more likely to purchase private health insurance. And higher household income and expenditure, higher education level are more associated with the increased probabilities of private health insurance purchases. This results imply that the expanding of private health insurance market could widen the gap between the have and have-not in terms of equal health care accessibility.