In this paper, we propose the development of a web-based, intelligent instruction system to help elementary school students for mathematical computation. We concentrate on the intelligence facilities which support diagnosis and advice. The existing web-based instruction systems merely give information on whether the learners' replies are `correct' or `incorrect', and only offer evaluations of the learners' results in terms of points. What is needed is a web-based instruction system that diagnoses the learner's comprehension status, and provides cause: why did the learner make the error? Our system has a facility that analyses the learner's weak points and has the ability to diagnose the cause of the error, giving advice to the learners and more detailed error information than extant systems. By accumulating user behavior and analyzing the learner's responses, our system provides relevant, individualized information, along with advice for the learners.