1. An image forming apparatus comprising: a control unit, a first image scanning device, and a second image scanning device, wherein the first and second image scanning devices are configured to scan a same surface of a same sheet with an image formed thereon on a sheet-conveying pathway, the image forming apparatus is configured to adjust an image forming condition based on information acquired through the scan by the first and second image scanning devices, the first image scanning device is configured to scan only a part of an area in a main scanning direction; the second image scanning device is configured to scan an area across an image forming width in the main scanning direction; and the control unit is configured to detect an anomalous value based on the information acquired through the scan by the second image scanning device, and prompt cleaning of the first and second image scanning devices in response to detection of the anomalous value.