PURPOSE: To accurately and surely detect the size of an original irrespective of the hue and density of the original by providing brightness switching means which switch the brightness of the facing surface of the original to an original cover plate and performing pre-scanning before and after switching.
CONSTITUTION: Firstly, the first scanning is performed and picture data are measured after the facing surface an original is made to black by rotating rollers 16a and 16b of the plate 10. The measured picture data are stored in a memory 1 after digitization. Secondly, the second scanning is performed after the back of the original is made to white by rotating the rollers 16a and 16b and measured picture data are inputted to a subtractor 2 after digitization. Simultaneously, the input picture A measured at the time of the first scanning is read out from the memory 1 and inputted to the subtractor 2. The absolute value picture C of the difference between the two pictures is binary coded through a comparator 4 and the size of the original is detected based on the binary coded picture. Therefore, sizes of originals can be detected accurately from any kinds of originals.