The expansion of the military alliance into the cyber domain raises new problems. In particular, the issues of physical response to cyber attacks and whether the alliance should address them are one of the most fundamental and key questions. Under the premise that the existing international law is applied to the cyber space as it is, If an attack on the cyber space meets the requirements in terms of ‘scale and effect’ reaching the level of an ‘armed attack’, it is possible to respond physically at the level of self-defense. It was also confirmed that under the same conditions, casus foederis of the alliance treaty occurred. The problem was the judgment of “use of force” and “armed attack,” but in the absence of clear standards under international law, it is reasonable to view this problem as a political issue rather than a technical issue. The question of whether to respond to cyber attacks physically or jointly within the framework of alliance is entirely a matter of political judgment of the parties.