A digital Y signal compatible with a plurality of pixels in one frame unit is divided into a plurality of signal level regions for each constant signal level range by means of a histogram generator circuit, a rate of pixels included in each signal level to all the pixels is detected, and a histogram is generated. In addition, a correction coefficient is set, and the correction coefficient is stored in a correction coefficient storage circuit. In a computing circuit, a correction value relevant to a signal level in each signal level region is calculated in accordance with the histogram and correction coefficient, and input and output characteristics of the digital Y signal are adjusted on the basis of the calculated correction value. The digital Y signal is gamma-corrected by a Y-.gamma.correcting circuit in accordance with the input and output characteristics adjusted by the computing circuit.