Image processing apparatus and method capable of performing high-speed printing while suppressing an increase of memory size are provided. The image processing apparatus comprises: a page processing unit to perform processes of a plurality of pages in parallel; a load deciding unit to detect a load level in the page processing unit, for each page; and a controlling unit to control the page processing unit depending on a detection result by the load deciding unit, wherein the page processing unit assigns a thread of an edge extraction process to one or more image data in the page to be processed, and synthesizes an edge of each image data extracted by the thread, and the controlling unit comprises a thread priority order designating unit configured to designate priority order of the thread assigned by the page processing unit, depending on a result of the detection by the load deciding unit.