An image forming apparatus to generate image data from an original document and to form an image includes a photoelectric conversion unit to sequentially convert an image of the original document scanned in a sub-scanning direction into image signals made of a plurality of pixels constituting one line in a main scanning direction, a white reference plate which is white reference for the image signals, a white reference signal generation unit to generate a white reference signal from image signals obtained by photoelectric converting an image for a given number of lines from a given read start position of the white reference plate in the sub-scanning direction by the photoelectric conversion unit, a shading correction unit to correct the image signals of the original document image photoelectric converted by the photoelectric conversion unit based on the white reference signal, and a read start position acquisition unit to calculate and acquire the read start position on the white reference plate, and the read start position acquisition unit includes a start position calculation unit to detect a portion of the white reference plate where brightness is within a given range and to calculate the read start position.