This paper is to study the system and legal relationship of illness insurance. The Korean Commercial Act Article739-2 stipulates "Any insurer of a contract of illness insurance shall be liable to pay the insured amount and other benefits if a peril insured against causing a illness occurs", and the Article739-3 says "The provisions concerning life insurance and accident insurance shall apply mutatis mutandis to an illness insurance." There are closely related between Illness insurance and public medical insurance. According to the introduction of public insurance in Korean medical market, the accessibility to medical service for citizens in a blind spot of medical risk has improved and the size of medical market has been expanded. Therefore in this Paper, I enunciate some suggestions of the reasonable relationship to the debate about the legal nature of illness insurance. The issue on the introduction of private medical insurance is in fact embodying a dual conflicting objective of public spirit in medical service and efficacy in service delivery, the relative utility value of which is actually determined by the writer``s personal preference. This study tries to analyze the coverage and the legal relationship of the illness insurance, and propose the reasonable conclusion.