There is described an image forming system constituted by an image forming apparatus and an external apparatus. The image forming apparatus includes a storing section to store image data, an image forming section and a controlling section to control the image forming operations. While, the external apparatus includes a data generating section to generate a plurality of print data sets, which are different from each other with respect to at least a part of each print data set, a selecting section to select selected-image data from the image data and a transmitting section to transmit the plurality of print data sets and selecting information indicating the selected-image data to the image forming apparatus. Based on the selecting information, the controlling section controls the image forming section so as to couple the selected-image data with each of print data sets to output coupled-image data.