An artificial retina system for improving contrast sensitivity, comprising: an artificial retina which is configured to be installed under a retina and includes a plurality of photodiode cells; and a microcomputer configured to compare a magnitude of an electric signal output from a photodiode in each of the photodiode cells with at least two reference values, and to amplify or attenuate the electric signal output from each of the photodiode cells according to the result of the comparison, wherein each of the photodiode cells includes: the photodiode configured to convert light introduced from outside into an electric signal and to output the electric signal; a copier configured to copy the electric signal output from the photodiode; an amplifier configured to amplify or attenuate the electric signal output from the copier; and a stimulation electrode configured to stimulate the visual cells with the electric signal output from the amplifier, wherein the microcomputer is configured to adjust a gain of the amplifier to amplify or attenuate the electric signal output from the photodiode cell, and wherein the microcomputer is configured to store a gain value corresponding to a value which is equal to or greater than one of the reference values and another gain value corresponding to a value which is less than another reference value, and when the number of the reference values is N, the number of the gain values is N+1, wherein visual cells corresponding to each of the photodiode cells are stimulated with an electric signal controlled by the microcomputer. an artificial retina which is configured to be installed under a retina and includes a plurality of photodiode cells; and a microcomputer configured to compare a magnitude of an electric signal output from a photodiode in each of the photodiode cells with at least two reference values, and to amplify or attenuate the electric signal output from each of the photodiode cells according to the result of the comparison, wherein each of the photodiode cells includes: the photodiode configured to convert light introduced from outside into an electric signal and to output the electric signal; a copier configured to copy the electric signal output from the photodiode; an amplifier configured to amplify or attenuate the electric signal output from the copier; and a stimulation electrode configured to stimulate the visual cells with the electric signal output from the amplifier, wherein the microcomputer is configured to adjust a gain of the amplifier to amplify or attenuate the electric signal output from the photodiode cell, and wherein the microcomputer is configured to store a gain value corresponding to a value which is equal to or greater than one of the reference values and another gain value corresponding to a value which is less than another reference value, and when the number of the reference values is N, the number of the gain values is N+1, wherein visual cells corresponding to each of the photodiode cells are stimulated with an electric signal controlled by the microcomputer. the photodiode configured to convert light introduced from outside into an electric signal and to output the electric signal; a copier configured to copy the electric signal output from the photodiode; an amplifier configured to amplify or attenuate the electric signal output from the copier; and a stimulation electrode configured to stimulate the visual cells with the electric signal output from the amplifier, wherein the microcomputer is configured to adjust a gain of the amplifier to amplify or attenuate the electric signal output from the photodiode cell, and wherein the microcomputer is configured to store a gain value corresponding to a value which is equal to or greater than one of the reference values and another gain value corresponding to a value which is less than another reference value, and when the number of the reference values is N, the number of the gain values is N+1, wherein visual cells corresponding to each of the photodiode cells are stimulated with an electric signal controlled by the microcomputer.