This invention is directed to reduce a deterioration in image quality after thinning operation when pixels are read out upon thinning. An image sensor is designed such that different gains are applied on a pixel basis. After periodical gains are applied to the respective pixels, the pixels of the same colors are added and averaged to obtain a low-pass filter effect before thinning processing for the pixels in the sensor, thereby preventing the occurrence of aliasing (moiré) due to thinning.