1. A luminance gradation correcting apparatus comprising: a masking part for allowing only a luminance signal of a pixel in a predetermined detecting range in the vertical direction in an image shown by an input luminance signal to pass; a histogram memory for forming frequency data at each luminance level of the luminance signal outputted from said masking part at every predetermined period and storing said data; a correcting part for correcting the luminance level of said input luminance signal based on the frequency data in said histogram memory; and a detecting range setting part for obtaining an accumulation value of said frequency data with respect to a luminance signal in a portion of the luminance level in a range from a zero level to a non-image display color level in said input luminance signal and setting said predetermined detecting range based on said accumulation value.