1. An eye's optical characteristic measuring system, comprising an index projection system for projecting an index image on a fundus of an eye under testing, a photodetection optical system for guiding the index image toward a photoelectric detector, a simulation image calculating unit for calculating each of images of target images formed when a plurality of target images different in size are respectively projected on the fundus of the eye under testing based on a light amount intensity distribution of the index image detected on the photoelectric detector, and a visual acuity calculating unit for calculating a visual acuity value of the eye under testing, wherein said simulation image calculating unit calculates light amount intensity distributions in each of predetermined meridian directions of the images of the target images, and said visual acuity calculating unit detects a plurality of light amount intensity distribution values based on the light amount intensity distributions and calculates the visual acuity value of the eye under testing based on the plurality of light amount intensity distribution values.