A gap indicator device is rotatably locked to an auto document feeder openably and closably installed to a scanner device of an image scanning apparatus, is rotated by being pressed by the scanner device, and includes a pointer or a scale to indicate a state of an image reading gap, as a gap between a platen guide and an image reading portion, through which an original document passes. The pointer or the scale is located at a position where it can be visually recognized from above when a cover of the auto document feeder is opened.