A monitoring system detects an event happening to a detection target. The monitoring system includes an imaging device, storage, and a controller. The imaging device captures an image of an imaging area including the detection target to generate captured image data indicating a captured image. The storage stores therein a detection range in the captured image. The controller detects a change to the captured image in the detection range based on the captured image data. The detection range includes a detection target image exhibiting the detection target. Upon detecting a change to the captured image in the detection range, the controller changes the detection range so that the detection range encloses the detection target image.