Logic is a science of thinking. We understand the meaning of things and communicate each other by thinking logically, and has a rule of thinking in order to obtain universality. These are the principle of identity, the principle of contradiction, and the principle of excluded middle. There is also a syllogism called deductive inference, which draws a new judgment from two concepts or judgments. Logic is a science to demand thinking. But in meta-ethics, the universality is limited to the meaning of verbal proposition, because it reduces moral norm and behavioral norm into language problem, and explicate it in terms of linguistic analysis. After all, it is to analyse whether or not ethics has a meaning as a science. But ethics is a science on goodness, and belongs to the practical field of philosophy. What is the relationship between ethics and logical principle? The main point in this paper is whether or not the ethical proposition has universality or validity in accordance with logical principle. Accordingly, this paper is to investigate what kind of relations this moral law has to do with the principle of identity, the principle of contradiction, the principle of excluded middle, and syllogism, and is to demonstrate that moral law is a science which is subordinate to the principle of thinking in accordance with a logical principle. Although man is born equal, there is a difference in character (high or low) between men, and hence a difference in the quality of existence. By the way, man's character undergoes a change by practicing moral law, or by choosing moral law to the extent of participating in a community. What one leads a valuable life in life world means that one practices the common good by moral consciousness, and behaves like a human being under any circumstances. This practical consciousness is due to rational thinking, and ethics implies a law of knowledge and its practice.