1. An image processing device comprising: an inputter that receives a plurality of image frames captured chronologically; a gradation calculator, that calculates a first gradation distribution of a first image region in a target frame to be corrected among the plurality of image frames, and that calculates a second gradation distribution of a second image region corresponding to the first image region in a correcting frame used for correcting the target frame among the plurality of image frames; a similarity calculator that calculates a first similarity between the first and second gradation distributions; an image corrector that corrects the target frame using the correcting frame; and an output unit that outputs the target frame corrected by the image corrector, wherein the image corrector changes a usage ratio between the target frame and the correcting frame depending on the first similarity, and then corrects the target frame using the correcting frame.