Image scanning apparatus with dirt detection and removal of influence of the dirt

An image scanning apparatus includes: a reading unit that reads image data, which includes an image of a document transported along a transport path and an image at a scanning position of the image, from a storage area in synchronization with a reading synchronization signal; a detection unit that detects a dirt at the scanning position by processing the image data; and a delay unit that delays and outputs a period, during which the synchronization signal to be provided to the detection unit is valid, until reading of data of a document part included in the image data starts.