An image processing apparatus includes a gain correcting unit and a gain estimating unit. The gain correcting unit corrects a gain of a pixel output of a phase-difference detecting pixel. The gain estimating unit estimates the gain with which to correct the pixel output of the phase-difference detecting pixel. The gain estimating unit includes a provisional true-value calculating unit configured to calculate a pixel output of a provisional true-value calculating pixel, in accordance with a correlation of the pixel outputs between a base block including the phase-difference detecting pixel and a reference block set in a search area for which a block similar to the base block is searched.