Imaging forming apparatus capable of repeatedly reading out and printing stored image data

An image forming apparatus which is superior in operability and does not uselessly read image data is provided. When image data is once read and then a copy start key is pressed again, a determination is made as to whether an automatic document feeder has not been opened and a new original has not been set in the automatic document feeder from the end of a previous read operation to the next pressing of the start key. If the condition is satisfied, image data which is stored in a memory by the previous read operation is read out and printed. If the condition is not satisfied, an original image is read again.