The first reading start point D of a front contact image sensor 21 and the second reading start point F of a back contact image sensor 22 are managed using a previously measured sensor-to-sensor distance L. As a result, even if there is a difference in the distance between the sensors, the first and second reading start points D and F will still be a first margin length L1 before the image reading positions A and B of the contact image sensors 21 and 22. Because the beginning of the front image information FD2 and the beginning of the back image information BD2 contained in the back-side image data BD1 in the images captured by the contact image sensors 21 and 22 match, the front image information FD2 can be extracted based on the coordinates of the back image information BD2 contained in the back-side image data BD1.