A two-side image forming apparatus of the present invention is provided with a first sheet-transferring path, for use in one-side image formation, for transferring, to a printed sheet storage section via an image transcribing section, a sheet supplied from an unprinted sheet storage section; and a second sheet-transferring path, connected to the first sheet-transferring path, for supplying a tuned-over one-side printed sheet to the image transcribing section, the two-image forming apparatus being controlled so that a plurality of sheets are transferred concurrently in an overall sheet-transferring path when performing two-side image forming operation, the overall sheet-transferring path including the first sheet-transferring path and the second sheet-transferring path. With this arrangement, it is possible to attain a significant improvement in process efficiency.