PROBLEM TO BE SOLVED: To detect the incident angle of light to a photoelectric conversion part arranged below a document placing surface.SOLUTION: An image reading device includes a reading unit 22 which is provided below a document placing surface 211 and can read image data from a document placed on the document placing surface 211 and a photoelectric conversion part 231 provided below the reading unit 22 and outputting electric power corresponding to the light reception amount of light transmitted through the document placing surface 211 to be incident. The reading unit 22 is moved in a sub-scanning direction 25, to detect the position P5 of the reading unit 22 when a previously set change occurs in the electric power outputted from the photoelectric conversion part 231 and to detect the incident angle &thetas;1 of the light delivered from a light source L to the photoelectric conversion part 231, based on the position P1 of the photoelectric conversion part 231 and the detected position P5 of the reading
