A vehicle detection unit detects a plurality of vehicles traveling in an adjacent lane adjacent to a lane where the one vehicle is traveling, based on a sensor mounted on the vehicle. A speed information acquisition unit acquires speed information of a preceding vehicle traveling ahead of a target vehicle traveling right ahead of the one vehicle in the adjacent lane among the plurality of vehicles. A deceleration factor determination unit determines whether there is a deceleration factor in front of the preceding vehicle. A traveling state determination unit determines whether the target vehicle travels with the flow of traffic based on the speed information of the preceding vehicle and whether there is the deceleration factor.