A server includes: a registration unit that registers identification information for identifying a detection target in a storage unit; a detection target determination unit that determines whether the detection target is shown in an image captured on or around a road, based on the identification information; an abnormal behavior determination unit that determines whether the detection target exhibits an abnormal behavior that is different from a normal behavior of the detection target, when the detection target is shown in the image; and an alert transmission unit that transmits an alert when the detection target exhibits the abnormal behavior.