A pixel interpolating device includes a cycle calculation unit and an interpolating unit. The cycle calculation unit calculates a cycle in change of pixel values of pixels in a document image obtained by reading a document by first and second photoelectric conversion element arrays each having a plurality of photoelectric conversion elements. The interpolating unit extracts from the pixels such pixels that an interval between each pixel and an interpolated pixel corresponding to a position between the first and second photoelectric conversion element arrays corresponds to the cycle calculated by the cycle calculation unit. The interpolating unit interpolates a pixel value of the interpolated pixel based on pixel values of the extracted pixels.
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