Full Title: Provision Problem of Public Goods – Part 1- [Article No. 2.4.1]
日本語タイトル：公共財の供給問題- Part 1- [記事 No. 2.4.1]
Posted on June 15, 2026.
[Image]
(In Japanese)
Note: For a more detailed discussion of the Samuelson condition, particularly when expressed in terms of the marginal rate of substitution (MRS) and the marginal rate of transformation (MRT), please consult textbooks on public economics at the intermediate level or higher.
（注）サミュエルソン条件の，特にMRS（限界代替率）とMRS（限界変形率）を用いる場合の詳細については，中級以上の公共経済学のテキストをご覧ください．
Source: By Yoshida.
[Discussion]
- Image 1 が示すとおり，民間企業と政府の問題の基本構造自体は相似形である．しかし，政府が直面する問題には次の点において，民間企業が解いている問題よりも厄介な面がある．(1) 政府の意思決定に必要な各種情報は市場から直接的に得ることができない．(2) 政府が供給する公共財は大きな外部性を有する（社会に一つの公共財が供給されると，社会の全構成員がその財を利用できる）．
- 上記1(2)で示した特徴から，公共財の最適供給条件は Image 1 で示した「サミュエルソン条件」となる．サミュエルソン条件の左辺は社会の各構成員にとっての公共財からの限界効用の総和となっている．この左辺が形成される本質的な理由は，社会の全構成員が公共財を共同利用できる点にある．
- As shown in Image 1, the foundational structure of the problems faced by private firms and the government is fundamentally similar. However, the challenges confronting the government are considerably more complex than those of private firms in the following respects: (1) the diverse information required for public decision-making cannot be directly revealed through market mechanisms; (2) public goods possess a unique characteristic that generate substantial externalities—meaning that once a public good is supplied, its benefits are non-excludable and available to all members of society.
- Based on the characteristic described in Item 1 (2) above, the optimal supply condition for public goods is the “Samuelson condition” shown in Image 1. The left-hand side of the Samuelson condition represents the sum of the marginal benefits derived from the public goods by each member of society. The fundamental reason for this left-hand side is that these goods can be jointly consumed by all members of society.
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