1. An image processor for correcting an image, comprising: a reduced image forming unit forming a reduced image by reducing an original image; an image filter acquiring unit acquiring a reduced image filter for the reduced image and an original image filter for the original image, the image filter acquiring unit generating either one of the original image filter and the reduced image filter based on the other one of the original image filter and the reduced image filter; a reduced image average value calculating unit calculating an average peripheral value for each pixel in the reduced image using the reduced image filter; a reduced image reflectance acquiring unit finding a reflectance value for each pixel in the reduced image from a value of the each pixel and the average peripheral value of the each pixel; a normalization parameter setting unit tabulating frequencies of each reflectance value within the entire range of reflectance values found by the reduced image reflectance acquiring unit for all the pixels and setting a normalization parameter; an original image average value calculating unit calculating an average peripheral value for each pixel in the original image using the original image filter; an original image reflectance acquiring unit finding a reflectance value for each pixel in the original image from a value of the each pixel and the average peripheral value of the each pixel; a normalizing unit normalizing the reflectance value for each pixel in the original image based on the normalization parameter; and a correcting unit correcting a value for each pixel based on the normalized reflectance value for the each pixel in the original image.