1. An image processing apparatus comprising: a decimating unit 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; a similar pixel extracting unit configured to 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; a first correction-candidate-value calculating unit configured to calculate a correction candidate value used to correct a pixel value, based on pixel values of the similar pixels; a second correction-candidate-value calculating unit configured to calculate a correction candidate value for each of the pixels decimated by the decimating unit, based on the correction candidate value calculated for each of the pixels contained in the decimated image; and a correcting unit configured to correct a target pixel value of a target pixel to be corrected in the target image, based on the correction candidate value calculated for the target pixel by the first correction-candidate-value calculating unit or by the second correction candidate value calculating unit.