An image forming apparatus that enables to suppress the cost rise caused by increasing the number of sensors for original size detection and suppress the degradation of user operability. Paper sizes of papers are registered on a plurality of paper supplying units in advance. One of the registered paper sizes is determined as the original size if the original size cannot be detected by a detecting unit. An image of the original is formed on a paper, which corresponds to the original size determined by the determining unit and is fed from the paper supplying unit.