An image scanning apparatus includes a controller configured to perform a light quantity adjustment determining process to determine whether a detection light quantity needs to be adjusted, based on adjustment determination values, in a black-white detectable position, a light quantity adjusting process to adjust the detection light quantity to maximize an adjustment difference value between a gradation value generated by scanning a white portion of a reference member and a gradation value generated by scanning a black portion of the reference member, a threshold calculating process to calculate a detection threshold based on a black value and a white value, and a reference position detecting process to detect a reference position by comparing, with the detection threshold, gradation values generated by scanning the reference member while illuminating the reference member with the detection light quantity.