Image reading apparatus and shading correction method for switching shading correction based on external light

The image reading apparatus includes a reading unit, a reading region in which a first region in which a member that suppresses entry of external light is disposed and a second region other than the first region are arranged in a main scanning direction of the reading unit, and a controller, wherein the controller calculates intensity of external light in the second region based on a reading value in the reading region, wherein the reading value is obtained by the reading unit, determines whether the intensity of the external light exceeds a threshold value, performs a first shading correction on a reading value of a document transported to the reading region when the intensity of the external light exceeds the threshold value, and performs a second shading correction on the reading value of the document when the intensity of the external light does not exceed the threshold value.