1. A perimeter comprising: a portion for forming data concerning a visual field characteristic distribution in such a way that interpolations are made between tested values of a subject's eye which are obtained by exhibiting a plurality of targets, the visual field characteristic distribution including estimated values; and a portion for displaying an unreliable distribution which denotes a domain estimated to be distant from said tested values on the basis of both said data concerning the visual field characteristic distribution and data which include the test values, on a distribution of an estimated neighborhood of the tested values.