A point at which the luminance varies between successive frames is detected with respect to an input image signal whose one-frame period is represented by a given gradation level. In a frame in which the luminance has just varied (i.e., in a post-change frame), the gradation level is corrected so that the lack of response of an image display panel is remedied (overshoot driving). In at least one embodiment, a sub-frame signal is generated in accordance with the signal whose gradation level has been corrected, so that an output gradation level to be outputted to the image display panel is obtained.