An image reading apparatus capable of generating accurate shading correction data while removing noise. A shading reference plate is arranged at a location having different optical distance between the location and an image sensor in an image reading unit from optical distance between original reading position and said image sensor. The shading reference plate is read by the image reading unit whereby secondary reference data is acquired, and a reference sheet is read which is put on a position substantially identical to the original reading position whereby primary reference data is acquired. Primary and secondary correction data is generated based on the primary and secondary reference data, and correction coefficient data is generated based on the primary and secondary correction data. Shading correction data is generated based on read data of the shading reference plate and the correction coefficient data. The shading correction of the images of original read by the image reading unit is performed based on the shading correction data.