PURPOSE: To convert a luminance signal into a digital signal without decreasing gradations regardless of whether the mean luminance level (APL) of the luminance signal is high or low.

CONSTITUTION: A voltage output which varies with the APL of the luminance signal is generated and supplied to a terminal of an A/D converting circuit 1 which determines the mid-point of a reference voltage. For a video signal with low APL, the mid-point potential of the reference voltage is raised to increase the contrast of the part of the low APL and increase gradation expressing performance. For a video signal with high APL, reverse operation is performed to increase the gradation expressing performance.