Limited coverage for health care services of National Health Insurance(NHI) in Korea has been ongoing policy issue but additional NHI financing through raising contribution or taxes in order to improve coverage faces substantial obstacles. Private health insurance(PHI) is often considered as an alternative financing source to improve coverage. Recent reform that attempted to stretch the role of PHI allowed life insurance companies to provide complementary PHI, indemnity plan which will pay for uncovered services by NHI and out-of-pocket spending for covered services. Although complementary PHI may relieve financial burden of patients, it may significantly raise NHI spending as well as total health expenditure since little out-of-pocket spending may increase utilization of health care. So far, there has not been enough discussion about concerns of potential adverse effect resulting from extended role of PHI. This study investigated potential increase of NHI spending followed by extension of complementary PHI through sensitivity analysis. The amount of NHI spending for services that would be covered by complementary PHI was calculated using 2005 NHI statistics and expected complementary PHI enrollment rate by age and sex. Expected utilization increases were obtained based on price elasticities$(-0.2{sim}-0.5)$ from previous studies and expected coverage rate$(50{sim}80%)$ of complementary PHI and then converted to monetary figures. Because coverage rate of complementary PHI has not been determined yet, we employed the sensitivity analysis using coverage rate of $50{sim}80%$. Findings demonstrate that additional spending for health care services is expected to be $426{sim}1,702$ billion won, corresponding amount payed by NHI $298{sim}1,192$ billion won. In conclusion, since complementary PHI may raise NHI spending significantly, there should be an agreement whether this additional cost would be accountable and acceptable in our society. Potential inefficiency resulting from extended role of complementary PHI should be considered since public and private financing do not operate in isolation and there should be more discussion on proper role of PHI in Korea.