The purpose of this study was to examine the role of learners' height judgment in solving the "same perimeter problem (SPP)": a task that asks whether the area of a transformed parallelogram in which only the angles have been changed is equal to the area of the original parallelogram. In previous studies, it was assumed that participants had no issues about the height judgment in solving the SPP. The experiment revealed that the height judgment in the SPP was difficult for more than half of 49 undergraduate students, the participants, and that more than half of the participants who misjudged the area and the height could get correct judgments if presented with an auxiliary line. These results suggested that incorrect judgment of area in the SPP caused by incorrect judgment of height.