In an 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.