The present invention relates to a method for enhancing an image quality by saturation. The method includes the step of: determining a minimum gain value `SatGainMin` and a maximum gain value `SatGainMax` of a gain value `PGain` of a pre-amplifier; defining a first to a third ranges based on the `SatGainMin` and the `SatGainMax`; determining whether a currently used saturation value `USat` is equal to or greater than approximately 1; determining whether the `PGain` is less than the `SatGainMin`; performing saturation by using a value in the first range as an actual saturation value `ASat` when `PGain`< `SatGainMin`; determining whether the `PGain` is equal to or less than the `SatGainMax`; performing saturation by using a value in the second range as the `ASat` when `SatGainMin`.ltoreq.`PGain`.ltoreq.`SatGainMax`; and performing saturation by using a value in the third range as the `ASat` when `PGain`>`SatGainMax`.