An image reading apparatus including a light source for illuminating an original, a white member which is illuminated by the light emitted from the light source, a photoelectric converting element for receiving the reflection light from the original and the reflection light from the white member, a memory for storing a reference output of the photoelectric converting element which is obtained by receiving the reflection light from the white member, a shading compensation device for compensating an output of the photoelectric converting element which is obtained by receiving the reflection light from the original in accordance with the reference output, and a rewriting device for rewriting the reference output to be stored in the memory, wherein the rewriting device rewrites the reference output at a timing according to a fluctuation in light amount of the light source.