1. An image processing apparatus comprising: a scanning unit for scanning a scanning zone in which a plurality of documents may be placed, and for acquiring an overall image of the scanning zone; an image detection unit configured to detect a plurality of image areas in the overall image each corresponding to a respective one of the plurality of documents in the scanning zone, wherein the plurality of image areas is detected by using a predetermined parameter; a size detection unit configured to detect a size of each of the plurality of image areas; a setting unit configured to set a reference area size as the sizes of the plurality of image areas; a calculating unit configured to calculate the difference between each of the sizes of the plurality of image areas and the reference area size; and a decision unit configured to decide that an image area detected by the image detection unit has been detected correctly if the difference is equal to or less than a preset threshold value, and to decide that an image area detected by the image detection unit has not been detected correctly if the difference exceeds the preset threshold value, wherein responsive to a decision that the image area has not been detected correctly, the predetermined parameter is changed, and operations of image detection by the image detection unit, size detection by the size detection unit, and calculation by the calculating unit, are all repeated until the difference is equal to or less than the preset threshold value.