The purpose of this study was to investigate the effects of learners' existing mis -rules (ru) on area learning. It was inferred that schoolchildren had a ru that if the perimeter of a figure is longer, the area of it is larger (perimeter-ru). The relations between the presence of this ru and the existent education of area were researched. The results were: (i) More than fifty percent of the children in each grade judged areal size in terms of the ru.(ii) The percentage of the ru-response rapidly increased after area learning. These results suggested that (a) the present education of area could not reconstruct the perimeter-ru to the correct rule (ru), and (b) rather, might strengthen misjudgments caused by the perimeter-ru.