Integration of Intelligent Tutoring System (ITS) and World-Wide Web (WWW) technology produces novel educational systems. This paper describes architecture of WITS (WWW based ITS) especially its individualized tutoring mechanism and illustrates features of our system, a WITS for guiding differential calculations at high school. The basic structure of WITS follows a standard ITS. All modules of the WITS are constructed on a WWW server except interface, which is implemented with a WWW client such as Netscape Navigator^<TM> or NCSA Mosaic. CGI (Common Gateway Interface) and FOF (Fill-Out Form) facilities of the WWW take an important role in flexible and individualized tutoring. They provide dynamic response to a user's action. We have applied buggy modeling method in ITS technology for user modeling mechanism in the WWW framework. Combining CGI and FOF facilities of the WWW with user modeling technique on an ITS accomplishes "User Identification, " "User (Student) Modeling" and "Adaptive Tutoring." Distinctive features of our system are (1) user's (student's) error origin identification based on buggy model, (2) student's understanding state classification based on his score and frequency of the bug, (3) teaching paradigm selection and guidance message generation based on the student's state, and (4) KR message generation which depends on the situation in the WWW framework. The advantages of the WWW for an ITS are that a user can access to the system from anywhere on the Internet without any special interface programs. Collective data on the server enable statistical analysis for ststem evaluation and improvement. Although the WWW framework causes some limits especially in its interface, it brings an ITS great benefit.