A CCD photoelectrically converts image information obtained from optically reading an original image, line by line, and outputs an image signal. Several of the photoelectric sensors of a CCD of the CCD portion form an optical black portion. A black shading correcting portion corrects the image signal using a black reference level. The black reference level is obtained from the optical black portion of the CCD for each line during an operation of the reading of the original image. The black reference level used by said black shading correcting portion for each line is obtained using black reference values, each of the black reference values being data of the optical black portion for a respective one of a plurality of lines.