1. A gradation control apparatus comprising: a coring module configured to change a coring amount of a video signal such that the coring amount increases as the degree of movement of the video signal becomes higher; a smoothing module configured to perform a gradation smoothing process on the video signal; and a parameter controller configured to obtain a frame difference histogram as the degree of movement of the video signal, weight the frame difference histogram so as to have a characteristic in which a frequency increases as a difference value becomes larger, and change an effect parameter for the gradation smoothing process according to a sum of weighted frequencies.