This article discusses whether or not the United Nations Convention on Contracts for the International Sale of Goods [hereinafter, CISG] is applicable to set-off cases. After examining of the view indicates that the issue of set-off lies inside the CISG’s scope of application, this article concludes that the set-off falls out of the field of application of the CISG, even in cases where mutual claims arise from contracts that are governed by the CISG(Ⅱ in this article). Therefore set-off is dealt with by the recourse under the applicable domestic law, the article discusses the question of which law should be applied to a case in which the claims are governed by the different laws (Ⅲ in this article). The Rome Convention (1980) does not contain a general conflict-of-laws rule of set-off. Although it is needless to say that set-off can be qualified as a means of extinguishing the obligation, Article 10(1)(d) of the Convention does not reflect the difficulties that is inherent in the set-off mechanism which is applicable to the two obligations subject to different jurisdictional laws. In the view of substantial law, the rules that govern choice-of-law vary depending on the legal system of that jurisdiction. Where set-off can be affected by an extrajudicial and unilateral declaration, the law of the claim against the person declaring set-off is applicable (Germany and the Netherlands). If set-off is in principle effective ipso iure, the conflict-of-laws rules provide a cumulative application of the laws of both claims (France and Italy). In case where set-off is traditionally characterized as a procedural relief, the lex fori governs the set-off (England). The article comes to the conclusion that the applicable law in that case is the law that governs the claim against what the defense of set-off is directed. This approach is simple to apply in countries where set-off has to be declared to be effective, and it is also in line with the approach shown in Article 16 of the EC Regulation on the Law Applicable to Contractual Obligations (Rome I).