多様で，ときに対立する価値観や世界観をもつ人々が共に戴くことができ，自由な共生の媒介となりうる公正な制度のあり方を，私たちはどのようにして探求できるだろうか．ジョン・ロールズの政治的リベラリズムの中心概念である公共的理性は，この問いへの有力な回答である．公共的理性は，社会の基本的諸制度のあり方をめぐる議論が市民にとっての公共的価値の観点からなされることを求める．このような観点から支持すべきものとして人々が支持しうる制度が，真に公共性を備えた制度だと考えるのである．本稿は，ロールズによる公共的理性への呼びかけに応じる試みの一環として，基本的社会制度のなかで重要な位置を占める家族制度，なかでも婚姻制度に焦点を当て，オルタナティブな婚姻形態の制度的包摂にかんする公共的理性を提案する．

具体的な考察対象は別氏婚および同性婚である．近年提案されてきたこれらオルタナティブにかんしては従来からさまざまな擁護論が展開されてきたが，公共的理性の観点からとらえた場合には，問題が少なくない．本稿は，人格的ケア関係および次世代の適切な育成という二つの公共的価値の観点から各オルタナティブがもつ意味を改めて検討し，公共的理性が婚姻制度へのそれぞれの包摂を支持するかどうかを検討する．結論は肯定的である．

In a liberal political society where individuals adhere to diverse and incompatible worldviews, how is it possible to find a just institutional arrangement that can be sincerely supported by all as a mode of living together? Public reason, a central ideal of John Rawls’ political liberalism, points to a promising approach. Public reason demands that our arguments about society’s basic institutional structure be based on public political values that rest on the perspectives of free and equal citizens. This paper is a response to his call and presents a public reason argument for the institutional inclusion of alternative marriage forms. The institution of family is an important part of society’s basic structure, and marriage is at its core.

This paper focuses on two alternatives, separate-surname marriage and same-sex marriage. Separate-surname marriage has been a controversial issue in Japan, a country that continues to require married couples to use a common surname, while same-sex marriage has recently emerged as a political agenda. Although numerous arguments for the institutional inclusion of alternative forms of marriage have been developed, most are not based on the use of public reason in a pluralistic liberal society. This paper reexamines eachof the above alternatives based on two public political values; the value of personal caregiving relationships and the value of orderly reproduction, to see whether public reason can support their inclusion as social institutions. The concluding section responds to possible criticisms of this approach and defends the ideal of public reason in a broader context.