Effective information services could be provided for users by investing meta data in information sources and making software agents understand it. In this paper, we make better use of a collection (ontology) of meta-data in the Semantic Web technology for the design of an e-learning system that enables to provide learners with a more effective education support. The learning domain is cryptography in network security. We define various concepts in the cryptographic domain and the relations among them as the ontology, and propose a way of utilizing it in the learning process. The strength of our ontology-based e-learning system is demonstrated through application examples of the prototype system that we have been developing.