Provided is a method of automatically setting image quality, which maintains a brightness and a contrast of an image regardless of a status of an input signal. The method includes: (a) setting a variable range of brightness and a variable range of contrast; (b) converting a brightness signal included in an input video signal into a digital signal and recording the digital brightness signal in a plurality of registers according to brightness level; (c) changing a brightness value or a contrast value of an output video signal by comparing values of a first register, second register, third register, and fourth register among the registers to predetermined values; and (d) if the changed brightness value is in the variable range of the brightness and the contrast value is in the variable range of the contrast, determining that the compensation of image quality is complete, and returning to step (b).