In an image reading apparatus having a light source for illuminating a document, an image sensing element for outputting an electrical signal in accordance with an input light quantity, a first reference member, and a second reference member, shading correction data is acquired. A time since the light source is turned on is measured, and when the measured time does not reach a predetermined time, shading correction data is acquired by a first method using the first reference member. When the predetermined time has elapsed, shading correction data is acquired by a second method using the second reference member.