A time required for a copying operation is shortened in making duplex copies from simplex documents. In a copying machine provided with a recirculating document feeder for feeding documents one by one from a document container to an exposure region and returning the exposed documents to the document container, when the documents are circulated prior to the copying operation so as to count the number thereof, the copying operation is started immediately in the case where the number of the documents is detected before a first document passes the exposure region.