Cyber conflicts are an important security challenge, but the applicable legal regime remains ambiguous. Especially, low-intensity cyber conflicts, which do not amount to armed conflicts under international law, are difficult to handle legally because the boundary between conflicts and crimes is ambiguous. Domestic debates on the legal response to such cyber conflicts so far have mainly focused on the applicability of self-defense under Japan’s extreme pacifist constitution. However, applying self-defense to low-intensity conflicts is quite difficult under the constitution, and further progress in the debate is unlikely. This study proposes specific ways to respond to cyber conflicts by utilizing countermeasures as a new legal framework. In the first part, after touching on an overview of the application of international law in cyberspace, this study will show the advantages of countermeasures under low-intensity cyber conflicts. In the latter part, through some scenario analyses, this study will clarify concrete ways of how to apply countermeasures, foreseeable problems, and how to respond to them as conclusions. The rationale for this study was mainly based on a literature review, including previous studies, and the scenario study method was also used to draft the conclusions.