The proposed apparatus is designed for being capable of recovering even when the overlapped transport of insert sheets has occurred without stopping the apparatus, thereby providing an improved utility to the users. The apparatus includes a first sheet feeder for sequentially separating blank sheets into individual sheet, an image forming device for forming images on the blank sheets, a transport device for waiting the blank sheet to be supplied to the image forming apparatus at predetermined time, a second sheet feeder for sequentially separating the insert sheets into individual sheet, an overlapped transport detecting device for determining whether or not the insert sheets have been transported in overlapped condition, and a controller for discharging all the insert sheets of a set involved in the overlapped transport onto a predetermined place, finding the first insert sheet of a next set of insert sheets, and discontinuing the drive of the transport device.