The invention discloses a gamma adjusting apparatus for adjusting the gray level of each pixel of an image signal. The gamma adjusting apparatus includes a measuring unit, a first processing unit, a second processing unit, and a third processing unit. The measuring unit measures the lightness of N reference gray levels. The first processing unit normalizes the lightness of the N reference gray levels to generate N normalized reference values. The second processing unit compares the N normalized reference values with a standard gamma curve to generate N adjusted gray levels. According to the N reference gray levels and the N adjusted gray levels, the third processing unit sets an adjusting rule. According to the adjusting rule, the gamma adjusting apparatus is capable of adjusting the gray level of each pixel of the image signal to comply with the standard gamma curve.