This study was to explore the effects of using calculator in learning decimal numbers. The subject of this study was a sixth grader who had misconception of 「fraction rule」 in comparing the largeness of decimal numbers. The author adopted semi-structured interview to conduct this study. The teaching treatment was using the calculator to do「repeated adding/subtracting 0.01」and 「multiplying 10 or dividing 10」activities. The results showed calculator-teaching activities benefited for this study subjects. That is, he had overcome his original misconception after calculator-teaching activities treatment.