1. An image processing device comprising: an obtaining unit configured to obtain an input image; a first determining unit configured to determine a detection order that indicates an order of performing a detection operation for detecting whether an object is present with respect to partitioned areas into which the input image is divided, according to an existence probability that is set in advance for each of the partitioned areas and indicates a probability at which an object exists; and a detecting unit configured to perform, according to the detection order, the detection operation to detect whether the object is present in an area corresponding to a partitioned area subjected to the detection order in the input image, wherein the first determining unit determines the detection order so that, in a predetermined period of time, each of the partitioned areas is subjected to the detection operation at least once.