An image forming system includes a plurality of image forming apparatuses connected in series along a paper conveying direction. An image forming apparatus provided upstream in the conveying direction among the apparatuses prints one side of a sheet of paper and an image forming apparatus provided downstream prints the other side of the paper. Each of the upstream and downstream image forming apparatuses includes: an image forming unit; a storage unit; and a control unit to send a first information on a paper feed tray of the own image forming apparatus to the other image forming apparatus, receive a second information on a paper feed tray of the other image forming apparatus from the other image forming apparatus, make the storage unit store the first and second information, and make the image forming unit perform image formation based on the first and second information stored in the storage unit.