An image forming apparatus includes: an image carrier; an image-forming device including a toner and forming a toner image on the image carrier during a first running period; a detector detecting a toner quantity in a set period within the first running period; and a toner supplying device supplying the image-forming device with a toner according to the detected toner quantity. The apparatus further includes 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 detection plural times over a period longer than the period, thereby measuring a result stable time required to stabilize the result of the detection, and setting in the detector, as the period, a period over which the result of the detection is stable within the first running duration.