Proportionality is a general principle of law that directly derives from the idea of justice. It expresses the idea of "measure", the prohibition of excesses in the exercise of power and generally in the manifestation of any behavior. Historically, it has been associated with public law, in which it has had significant application as a criterion for controlling the restrictions imposed by state authorities on the exercise of constitutional rights. However, its application is not limited only to public law. The principle of proportionality runs through all areas of law and therefore applies to private law as well. In both private and public law, it functions as a mechanism for resolving conflicts between legal goods and interests. However, in every case of its application, whether as a criterion for controlling the content of contracts or as a criterion for the exercise of private law rights, it is not enough to characterize it as a "generally applicable principle" to justify the restriction of private action and behavior, especially when such action is an expression of constitutionally protected rights. It is necessary to seek the specific reason for the validity of the principle and to specify its content. The content assumed by the individual requirements of proportionality, namely suitability, necessity, and strict proportionality, varies according to the differences in conflicts between rights and interests that this principle is called upon to regulate. These two issues, the "whether" and the "how" of the application of the principle of proportionality to restrict the right to strike, are the subject of this study.