A modulating and demodulating section receives, from another radio device, a packet signal including at least position information of another vehicle in which the other radio device is mounted. A position information acquiring section acquires position information of the vehicle in which the radio device is mounted. An estimating section estimates a time taken for the vehicle and the other vehicle to encounter each other on the basis of these two pieces of position information. A deriving section derives a distance between the vehicle and the other vehicle on the basis of these two pieces of position information. A determining section provides notification of an encounter with the other vehicle in a case where the estimated time is equal to or less than a first threshold value or in a case where the derived distance is equal to or less than a second threshold value.