Image processing device, control method thereof, imaging device, and recording medium that determine a priority part from a plurality of detected parts

An image processing device that acquires an image and detects a subject. The image processing device includes at least one processor and a memory holding a program that causes the processor to function as a detection unit configured to detect a plurality of parts related to a same subject within the image, a determination unit configured to determine a priority part from the plurality of parts detected by the detection unit, and an extension unit configured to extend a region of the subject corresponding to a first part from the first part determined as the priority part in a direction of a second part that is different from the first part among the plurality of parts.