An image-data processing apparatus corrects flicker in each image-data frame. The apparatus includes a threshold generator that generates a threshold; an integrator that integrates values of extractedpixel data of a selected range in each frame on the basis of the threshold; a storage unit that stores integrated values of frames; an average calculator that calculates an average value of the imagedata on the basis of the integrated values; a gain calculator that divides the average value by an integrated value associated with a target frame, thereby calculating a reference correction gain; acomparator that checks whether pixel data in the target frame are in the selected range; a correction-gain output unit that outputs a correction gain on the basis of the comparison; and a corrector that corrects each piece of the pixel data in the selected range on the basis of the correction gain.