Disclosed is an image forming apparatus which generates print data based on control data for page print inputted from an external device and which forms an image based on the print data. The image forming apparatus includes a control unit having a plurality of arithmetic processing units, and an image forming unit for forming an image based on the print data which is outputted by the control unit. The control unit generates the print data by carrying out a rasterizing process of one page in band unit constituted by a predetermined number of lines based on the control data for page print and sequentially outputs the generated print data at output timings defined in a constant output interval, wherein the rasterizing process is controlled based on whether or not generation of print data is to be completed within the output interval for all of bands in the one page.