Misjudgement that areas are the same when their circumferences are identical is often made by pupils. The phenomenon has been interpreted by poor ability of discrimination between areas and circumferences and/or non-maturity of cognition in area. The present study, however, shows (a) that the phenomenon is not observed in less mature preschoolers, but emerges about the time of entering school,(b) that the origin of misjudgement is connected with acquirement of conservation,(c) that the phenomenon is caused by generalization of conservation legitimate in weight and volume, but not in area during transformation,(d) that conservation of area is supported by continuity of transformation at first, and (e) that circumferences, then, are extracted as the index of conservation through continuity and begin to show an influence on judgement in area.