To provide an image forming apparatus and its control method with improved productivity by minimizing the occurrence of intermittent operation.

In the image forming apparatus, print data received from outside are developed into image data printable on a transfer sheet, the image data are transmitted to a printer engine part forming images by an electrophotographic process, to create printed matter. When next image formation process is not started even after a set time from finishing of image formation by the printer engine part, given post-processing of the printer engine part is started, and the set time is made changeable.