An image forming apparatus comprises an automatic document feeder (ADF); a light source configured to irradiate the document conveyed by the ADF with light; a glass member which can be switched to a first mode in which the light from the light source is transmitted and to a second mode in which the light from the light source is reflected; a signal processing circuit configured to include an image sensor for respectively receiving the reflected light of the document and the glass member, read an image of the document in the first mode and generate a signal of a white reference value based on the reflected light of the glass member in the second mode; and a shading correction circuit configured to correct, based on the signal of the white reference value, image data of the document read such that the brightness in the horizontal scanning direction is uniform.