An image scanning apparatus includes a distance detector configured to output a detection signal having a value corresponding to a distance in a main scanning direction from a particular position to a side end of a document sheet being conveyed, and a controller configured to store a first position of the document sheet in a conveyance direction when the distance detector outputs a detection signal having a first value corresponding to a first particular distance, store a second position of the document sheet in the conveyance direction when the distance detector outputs a detection signal having a second value corresponding to a second particular distance, and calculate a skew angle of the document sheet based on the first position and the second position.