A first image taken by a first camera device in the plurality of camera devices and first imaging environment information indicating a first imaging environment of the first camera device at a time of taking the first image is acquired. By using a parameter table that manages imaging environment information indicating an imaging environment at a time of taking an image previously by a camera device and a recognition control parameter indicating a detector corresponding to an imaging environment, a first recognition control parameter indicating a first detector corresponding to third imaging environment that is identical or similar to the first imaging environment indicated by the first imaging environment information acquired from the first camera device is selected from the recognition control parameters. The first image acquired from the first camera device is recognized by using the first detector indicated by the selected first recognition control parameter.