1. An image reading method of reading a mixed image and a decimated image from each input image among a plurality of input images which are captured, said image reading method comprising: determining, by a reading position determination unit, a reading position of a reading pixel of the mixed image and the decimated image for each of the plurality of input images, so that the reading position moves an amount which is different for each of the plurality of input images; reading, by an image reading unit, the mixed image from each of the plurality of input images at the determined reading position, by mixing signals of adjacent pixels; and reading, by the image reading unit, the decimated image from each of the plurality of input images at the determined reading position, by selecting signals of spatially-separated pixels.