The invention relates to an image reader, an auto document feeder, and an image forming apparatus. The invention has the object that even if the opposite guide part of the reading member is adhered with dirt, it is not influenced so as to obtain the accurate shading data. In the image reader, a reading member fixedly mounted in the image reader reads an image on a sheet conveyed into the image reader. A guide is disposed opposite the reading member to support the sheet with respect to the reading member. A guide moving member moves the guide with respect to the reading member in a main scanning direction in which the reading member reads the image on the sheet. A guide rotating member rotates the guide in a sub-scanning direction perpendicular to the main scanning direction. A controller controls the reading member to scan different positions on the guide moved by the guide moving member and rotated by the guide rotating member to generate shading data.