‘The effect of patent rights is limited territorially’. Should we take this principle of territoriality of patent rights for granted? No, some intellectual property law researchers in Japan argue. In recent years, several prominent researchers in Japan have argued that the territoriality principle regarding patent rights is merely a ‘policy decision’ that has no legal basis, and therefore, there is no problem in recognizing the effect of Japanese patent rights universally, disregarding the territoriality principle. I disagree. In this editorial, I shall first provide context by shedding light on the current state of the debate in Japan concerning the territoriality of patent rights. Thereafter, I will illustrate my position by primarily discussing territoriality as a substantive law principle, occasionally referencing conflict of laws when necessary. The discussion here focuses on the territoriality principle of patent rights among various intellectual property rights. Recently, the territoriality principle of patent rights has been a hot topic in Japan due to cases in which patent infringement through cross-border activities became an issue. Specifically, there are two cases between the same parties, Dwango Co. and FC2 Inc. Dwango alleged that its patents had been infringed by FC2’s construction of online systems and provision of programs related to video distribution. FC2’s server was located in the United States, so the question of whether FC2’s cross-border acts could constitute infringement of Japanese patent rights arose.