To enable a risk of collision between moving objects such as a pedestrian and a vehicle to be quickly and accurately determined in a roadside device,
when the roadside device receives a message including state information indicating that a pedestrian is in a risky state from a pedestrian terminal, the roadside device acquires position information of a vehicle present around the pedestrian in the risky state based on a result of detecting vehicles on roads by a radar, determines a risk of collision between the pedestrian in the risky state and the vehicle based on the position information of the vehicle, and when the risk of collision exists, transmits a message including alarm information indicating that the risk of collision exists to the terminal.