本稿の目的は、インターネットの特性を生かしたB2B市場の出現により現実的な問題となった買手寡占市場および買手寡占合意について、米国反トラスト法下における議論を参考にしながら、そもそも市場にどのような効果または弊害をもたらすのか明らかにし、独占禁止法の下でどのような分析方法を採用すべきか私見を示すことにある。当該目的を達成するため、本稿は、B2Bとは何か、競争法的観点からみたB2Bの特徴とは何か、なぜB2B上の取引について競争法的観点からの検討が現実的に必要であるのか、および取引市場としてのB2Bの特徴を明らかにするとともに、買手寡占市場が形成される市場構造とは何か、買手間の買手寡占合意についての経済学的見地からの分析結果はどのようなものか、買手寡占合意に対する米日における法的対応、買手が合併することによりB2B市場に発生する競争法的効果といった問題について検討する。

　B2B is a business-to-business market place that uses internet to connect each other business. It has gotten a lot more attention recently in Japan as well as in the U.S., because it is possible to lower the procurement costs of raw material and accomplish several pro competitive effects such as communication efficiencies. However, in spite of these pro transactional natures of B2B, it could also cause anticompetitive effects to market place, of which the paper analyzes, in light of the antitrust law in the U.S. and Japan, competitive effects due to creation of the oligopsonized market on B2B. Albeit it has been theoretically examined, the characteristic of B2B, i.e. communication efficiencies, makes creation of the oligopsonized market realistic. To deal with this issue, the paper first dissects the competitive effects of B2B. Then, the paper uses microeconomic theory and the antitrust law analysis in the U.S. and Japan, to explore the implications of the oligopsony and buying power, for antitrust policy. The paper offers a systematic treatment of the topic, demonstrating that whether or not the oligopsony power exists because of a dominant buyer or collusion among buyers, and it can cause social welfare losses analogous to those occasioned by monopoly. The paper creates the own economic model to describe what competitive effects the oligopzonized market will produce and how the society will be affected. The paper also discusses bilateral monopoly and offers a principled basis for distinguishing between socially desirable and undesirable cooperative buying. Moreover, the paper deals with the situation analogous to ologopzonized market, i.e. all or nothing deal to examine its competitive effects. In addition, the paper analyzes the legal response to an oligopsonized market and oligopsony agreement in the U.S. as well as in Japan in order to examine how an oligopsonized market, oligopsony agreement and agreement to realize all or nothing deal should be treated under the antitrust law.