1. An eye's optical characteristic measuring system, comprising a target projecting system for projecting a target image to an ocular fundus of an eye under test, a photodetection optical system for guiding said target image to a photoelectric detector, a simulation image calculating unit for calculating images of the target image to be formed when a plurality of target images of different sizes are independently projected to the fundus of the eye under test based on light amount intensity distribution of the target image detected at said photoelectric detector, and a visual acuity calculating unit having a predetermined threshold value and used for detecting light amount distribution characteristics from each of the light amount intensity distribution in a predetermined longitudinal direction of a plurality of target images calculated at said simulation image calculating unit and for calculating a visual acuity value of a person under test from an intersection of the light amount distribution characteristics and said threshold value.