Image forming apparatus

An image forming apparatus comprising:

• an image carrier on a surface of which an image is formed and carries the image;
• an image-forming device that includes a toner, forms a toner image on the surface of the image carrier with the toner while stirring the toner inside the image-forming device, performs forming the toner image within a first running duration, and stops forming the toner image after the first running duration;
• a detector that is attached to the image-forming device, detects a quantity of the toner included in the image-forming device within the first running duration, is set with a period of detection within the first running duration, and performs detection during the period which is set;
• a toner supplying device that supplies the image-forming device with a quantity of toner according to the quantity of the toner detected by the detector;
• and
• a period setting device that causes the image-forming device to stir the toner over a second running duration longer than the first running duration, and causes, during the second running duration, the detector to perform the detection a plurality of times over a period longer than the period, to measure a result stable time required for a result of the detection by the detector to stabilize, and to set in the detector, as the period of the detection within the first running duration, a period over which the result of the detection is stable within the first running duration.