A mobile object monitoring system that includes one or more terminal units and a monitoring unit that monitors an unknown mobile object around the one or more terminal units. Each terminal unit includes: an information acquisition component that acquires unknown mobile object information; a detected information generation component that generates a detected information set concerning an unknown mobile object; and a terminal-side communication component that transmits the detected information set. The monitoring unit includes: a collecting-side communication component that receives the detected information set; and a determination component that determines presence or absence of an unknown mobile object based on a collection of one or more detected information sets that have been received. The determination component establishes reliability of the collection of one or more detected information sets based on the number of detected information sets that indicate the detection of a specific unknown mobile object that is at the same location at the same time.