An image forming apparatus includes a rendering processing unit for generating rendering data by sequentially performing a rendering process on units of printing data corresponding to a job and storing the rendering data in a first area, an image processing unit for generating image data by sequentially performing an image process on units of rendering data in the page unit and storing the image data in a second area, an output control unit for outputting the image data in an order, and a memory full detection unit for detecting, if the output of subject units of the image data is incomplete when a specific time elapses since the rendering process on the subject units of the job is completed and only a part or all of the subject units of the image data are stored in the second area, that an image data storage amount exceeds an acceptable amount.