In regard to a secured original method, an image reading apparatus provided in an image forming apparatus reads a first original guide using an original reading portion and, based on a reading result thereof, changes a reading position of an adjustment sheet in a main scanning direction to enable detection of a difference between a brightness of the first original guide and a brightness of the adjustment sheet. In regard to a moving original method, the image reading apparatus reads a reading member using the original reading portion and, based on a reading result thereof, changes a reading position of an adjustment sheet in a main scanning direction to enable detection of a difference between a brightness of the reading member and a brightness of the adjustment sheet.