This study investigates the effects of the instruction type of instructors who are fully charged for underachievers on academic achievement in Seoul. In specific, this study focuses on the difference of effect on academic achievement by the instructors' teaching type. Major findings are like these. First, most of the fully-charged instructors for underachievers have taught their students in after school hours. But this type of instruction isn't the preference of them. Second, students who have been taught by their instructors only in after-school hours get the best academic achievement in mathematics. But the result of Korean is somewhat different. In specific, while third-grade students who have been taught in regular class by their instructor get a better result, forth-grade students who have been taught through the same manner get the worst result in Korean. Finally, in the case of fifth-grade, there is no differences of academic achievement by the instruction type. Their academic achievement is mostly determined by their previous academic achievement. From these findings, this study suggests that the policy that makes regular class instruction natural is needed. Also, it is suggested that early involvement is needed for underachievers.