A circuit acquires, as white correction data, read image data generated by reading an image of a reference member when light of each of three colors of a light source is sequentially emitted to the reference member; acquires, as original document image data, the read image data generated by moving the image sensor and by reading, with an image sensor, an image of an original document; determines, as an original document non-existence position, a position of a pixel at which all of the original document image red, green, and blue data have a same value; converts the original document image data to output image data by performing shading correction on the original document image data based on white correction data; and replaces the output image data at the original document non-existence position with a value closer to data indicative of white color than a value of the output image data.