1. A non-transitory computer-readable storage medium storing an image processing device that causes a computer to execute a process comprising: individually detecting, as a detection target candidate, a first portion of each of detection targets that appear in an image imaged by an imaging device, by using a first detection area corresponding to the first portion; detecting a detection target for a detection target candidate corresponding to a detection target that is not covered by another detection target and that is included in the detected detection target candidates, by using a second detection area corresponding to a second portion including the first portion and an additional portion of each of the detection targets; and detecting a detection target for a detection target candidate corresponding to a detection target, covered by another detection target and included in the detected detection target candidates, by using a third detection area that corresponds to a third portion including the first portion and the second portion, the detecting resulting when an amount of overlapping with the third detection area and the second detection area is less than a predetermined amount.