An image forming apparatus capable of feeding or transporting recording medium of different sizes through the image forming path. A discriminating device determines whether or not a recording medium has reached a predetermined position in the transport path and a detecting member detects the recording medium a predetermined time thereafter to identify the size of the recording medium. If the recording medium is detected within the predetermined time, it is identified as being of a particular size. When the medium is detected during a time period different from the predetermined time period, the medium is identified as being of a size smaller than the particular size. The image formation operation is controlled as a function of the size identification of the recording medium.
