漢斯．凱爾森的純粹法學無疑是當代影響深遠的法實證主義論。其影響並及於憲法學、國際法、政治學與社會學，二十世紀歐陸行政法學的發展亦不例外。本文分析純粹法學所建構的獨特行政法學方法論、法律關係理論以及行政法學上相對化的概念，以回顧凱爾森對近代行政法學發展的重要影響。

Hans Kelsen speaks about administrative law as a function of the state. The concept of administrative law for him does not mean law but science of law. Administrative law is merely law in that sense, like any other law, such as private law. Science of administrative law in Kelsen's understanding is therefore only an unspecific part of general science of theory of law. The question, which I answer here, is the relationship between pure theory of law and the development of science of administrative law in 20th century.