Image forming apparatus and storage medium

A non-transitory computer-readable storage medium having a program stored thereon for controlling a computer used in an image forming apparatus including an image former which includes, an image carrier which carries a toner image transferred to a sheet;

• a developer which applies a developing bias and which develops the toner image on the image carrier;
• a conductive rotating member which rotates while coming into contact with the image carrier;
• and a voltage applier which applies a voltage to the rotating member, wherein the program controls the computer to execute:
• an attached material detection mode which detects material attached to a surface of the rotating member,
• wherein, in the attached material detection mode, the voltage applied by the developer and/or the voltage applier are controlled so that the developing bias is a value between a surface potential in a region on a surface of the image carrier in contact with a portion in which attached material exists on a surface of the rotating member if a reference voltage is applied by the voltage applier, and a surface potential in a region on the surface of the image carrier in contact with a portion in which the attached material does not exist on the surface of the rotating member if the reference voltage is applied, and the developer is controlled to develop a toner image on the image carrier.