Fast spreading of web made we get easily the vast amount of information all over the world, but quantity of great information on the Internet space is giving much troubles to recognize change of information that users are interested soon justly. That is, users must connect and examine one by one to relevant site to detect change of web documents that changes from time to time. Therefore, the development of Robot which accomplish Information change monitoring function that sense automatically changed contents and inform to user is required. In this paper, we designed and implemented Web site contents change monitoring system, which notify-automatically the change of Web documents to users through alarm or E-mail if user defines target URL to do monitoring, monitoring condition, monitoring period etc. And we presented the method that structure and classify Web Documents to semantic units using HTML Tag. Also, we introduced the concept of virtual key to manage position of word to watch some change efficiently.