An image forming apparatus includes an image forming unit configured to form an image, a reader having a transparent member, the reader being configured to read an original through intermediation of the transparent member, a document feeder configured to feed the original in order to read the original by the reader, a reader image processor, and a controller. The controller is configured to control the image forming unit to form a test chart on a sheet, to control the reader image processor to detect the dust on the transparent member based on a result of reading the test chart, and to generate a conversion condition based on other signal values excluding an abnormal signal value caused by the dust on the transparent member.