Abstract: In-vehicle systems are in host vehicles. Each in-vehicle system transmits a vehicle information item indicating a present position, travel course, and size of the host vehicle to a server. Out of the host vehicles, detecting vehicles pass through a dangerous spot where a dangerous object is present. The in-vehicle system in each detecting vehicle transmits a dangerous object information item indicating an image and position of the dangerous spot, to the server. The server extracts warned vehicles expected to pass through the dangerous spot, from the present positions and travel courses of the host vehicles. The server determines a danger degree when passing through the dangerous spot, based on an image of the dangerous spot and a width of each warned vehicle, thereby preparing warning information and distributing it to the in-vehicle system of each warned vehicle.