1. An image interpolating unit configured to perform interpolation of a lost pixel, the lost pixel being located at a region in which a plurality of contact image sensors are coupled, and the contact image sensors each being provided with a plurality of image reading elements that are arrayed in line, the image interpolating unit comprising: a first interpolation pixel generator configured to generate, based on linear interpolation, a first pixel value on the lost pixel; a second interpolation pixel generator configured to detect, based on a difference total value, reference pixels of the lost pixel, and generate, based on the reference pixels, a second pixel value on the lost pixel, the difference total value being a total of differences between pixel values corresponding to pixels in a pixel group that is centered around the lost pixel and pixel values corresponding to pixels in respective pixel groups that are centered around a single pixel around the lost pixel; a reference pixel edge amount calculator configured to calculate edge amounts of the reference pixels; and an interpolator configured to insert, as an interpolation pixel value, one of the first pixel value and the second pixel value into a position corresponding to the lost pixel, based on the edge amounts of the reference pixels and a difference total value, the difference total value being a total of differences between pixel values corresponding to pixels in a pixel group that is centered around the reference pixels and the pixel values corresponding to the pixels in the pixel group that is centered around the lost pixel.