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, wherein the gradation calculator uses, among the plurality of image frames, at least one of an image frame captured before the target frame and an image frame captured after the target frame as the correcting frame, wherein, when the first similarity is at or below a first threshold, the image corrector employs a pixel value of the correcting frame as a first correcting value used for correcting the target frame, wherein, when the first similarity is above a second threshold, the image corrector employs a pixel value of the target frame as the first correcting value, and wherein, when the first similarity is above the first threshold and is at or below the second threshold, the image corrector calculates a pixel value by adding a pixel value of the correcting frame and a pixel value of the target frame with a weight factor depending on the first similarity, and then employs the calculated pixel value as the first correcting value.