In medical ethics, an appropriate national healthcare system that meets the requirements of justice in healthcare resource allocation is a major concern. Japan is no exception to this trend, and the pros and cons of introducing a two-tier healthcare system, which permits insured medical care services to be provided along with services not covered by social health insurance, have been the subject of debate for many years. The Supreme Court ruled in 2011 that it was valid for the government to ban mixing medical treatments and both the Ministry of Health, Labour and Welfare and Japanese Medical Association oppose the introduction of such a system. In this paper, we examine the main arguments against the introduction of a two-tier healthcare system and ideas that form their basis, which can be broadly divided into opinions based on the importance of equality and those derived from the logical conclusion that the outcomes would be detrimental. We point out issues and shortcomings of each standpoint and argue that the introduction of the system would not necessarily threaten equality and social solidarity in Japanese society, and that abuses of a self-pay tier would be controllable to a certain extent. Then, the authors—who do not actively promote the introduction of a two-tier healthcare system—conclude that the allegation that is completely devoted to one side and the theory that social healthcare is collapsing, which makes the public excessively nervous, cannot be accepted.