1. An image processing apparatus comprising: circuitry configured to: decimate pixels in a target image to be processed to obtain a decimated image containing a smaller number of pixels than the target image; extract similar pixels at each of which a similarity to a pixel of interest to be processed is equal to or greater than a first threshold, from a region containing the pixel of interest among pixels contained in the decimated image; calculate a first correction candidate value used to correct a pixel value, based on pixel values of the similar pixels; calculate a second correction candidate value for each of the pixels, based on the first correction candidate value calculated for each of the pixels contained in the decimated image; calculate a weight coefficient of a target pixel value and the first or second correction candidate value of the target pixel, based on the first or second correction candidate value of the target pixel, wherein the target pixel value is a value of a target pixel to be corrected in the target image; and correct the target pixel value of the target pixel, based on the first correction candidate value or the second correction candidate value, wherein the target pixel value is corrected by performing weighted addition of the target pixel value and the first or second correction candidate value by using the weight coefficient.