An image forming apparatus having a second storage device such as an HDD mounted thereon, when it receives an input of a job, determines a necessity of an input processing of image data into the second storage device, a necessity of an output processing of image data from the secondary storage device, and a timing of the input processing and the output processing in the job. Then, the image forming apparatus switches an output buffer, which is an area set in a primary storage of the image forming apparatus and is used for the output processing of image data from the secondary storage device, to a buffer for use in the input processing or to a buffer for use in the output processing in accordance with a result of the determination.