To provide a traffic monitoring apparatus capable of accurately determining the number of objects present in an area under surveillance without being affected by the difference in arrangement of those objects. Included is an object information acquiring device 1A for acquiring three-dimensional object information D related with an object H present in an area 8 under surveillance, and a determining device 2A for determining the number of objects based on at least a volume, which is one of a volume and a height of an object obtained from the object information D.