1. An image forming method using an image forming system that includes a plurality of image forming apparatuses each including a processing unit configured to perform a rasterization process of print data and an image forming unit configured to perform image formation on a sheet based on image data generated by rasterization process performed by said processing unit, the image forming method comprising steps of: (a) registering a print setting using attribute information that is necessary for the image formation performed by an image forming unit of a first image forming apparatus in a processing unit of a second image forming apparatus in advance; (b) determining whether or not any one of print settings of the print data coincides with the print setting registered in said step (a) when the rasterization process of the print data is performed by the processing unit of said second image forming apparatus; (c) causing the processing unit of said second image forming apparatus to generate the attribute information based on the print data when any one of the print settings of the print data is determined to coincide with the print setting registered in said step (a) in said step (b); (d) causing the processing unit of said second image forming apparatus to transmit the attribute information generated in said step (c) to the image forming unit of said first image forming apparatus through the processing unit of said first image forming apparatus together with the image data generated by the rasterization process performed by the processing unit of said second image forming apparatus and the print settings of the image data; (e) causing the image forming unit of said first image forming apparatus to receive the image data, the print settings, and the attribute information transmitted in said step (d) to said image forming unit of said first image forming apparatus through the processing unit of said first image forming apparatus; and (f) causing the image forming unit of said first image forming apparatus to perform the image formation for the image data received in said step (e) based on the print settings and the attribute information received in said step (e).