"What is Internet" was explained as an introduction to a series of twelve lectures. At first, this was defined as"Internet is an aggregate of computer networks all over the world connected with TCP/IP as a common language". Based on this difinition, it was explained that its developmental process was proceeded on by the linkage of industry government(including military authorities) university and extended acceleratively by the popularization of personal computer and the like, from its occurrence that was initiated by a research and development bearing its military use in mind. Also the history of Internet was outlined that has been accomplished a drastical growth by the development of WWW. Furthermore, a very small part of various application methods was introduced.