Detachable sheet feeding units are connected to an image forming apparatus. A transmitting unit transmits a feeding instruction via a signal line to a sheet feeding unit that is to perform sheet feeding. A first sheet detector, which is placed in the sheet feeding unit, detects a sheet that has been fed from the sheet feeding unit. A second sheet detector is provided downstream of the first sheet detector in terms of the sheet conveyance direction. If the feeding instruction is transmitted and a plurality of sheets are fed from the sheet feeding unit, an image formation controller determines whether to cause the image forming operation to continue or stop based upon whether a sheet-to-sheet interval of a plurality of sheets has been detected by the second sheet detector in a state in which the result of detection by the first sheet detector indicates presence of a sheet.