An image reading unit configured to rotate and read a front and a back of a document may deviate from an original reading position due to aged deterioration or an external physical impact and become incapable of reading an image accurately. An image processing apparatus rotates an image reading unit including a sensor in units of predetermined angles to read a white reference member, acquires luminance information about the white reference member at the respective predetermined angles of rotation, determines an angle at which the luminance information is greater than other pieces of the luminance information among a plurality of pieces of the acquired luminance information at the respective predetermined angles, and rotates the image reading unit based on the determined angle to correct a reference position of the image reading unit.