Even though there are extracurricular mathematics classes for gifted students in all levels of schools in Korea, teachers cannot conduct the classes properly because the contents of the textbook are not adequate for the purpose of the classes. So, what they tend to do in the classes is just drilling with many problems which have already shown up at university entrance examinations and various mathematics competitions. The purpose of this paper is to give an example of what the content should be in "Mathematics III" (an elective subject for the science high school students according to the fifth and sixth amendment of national curriculum) and to suggest how to design the extracurricular classes for gifted students. Extracurricular classes of the ordinary secondary school as well as the elective course for the science high school can be suitably designed with choices of topics in the contents of Mathematics III.