ASSIST DEVICE

An inter-vehicle calculation unit calculates a required inter-vehicle distance required between the own vehicle and forward/rear vehicles traveling in the lane to which the own vehicle moves during the lane change. The required inter-vehicle distance includes a first inter-vehicle distance and a second inter-vehicle distance. The first inter-vehicle distance is the distance required to allow the own vehicle or the forward/rear vehicles to adjust their velocity after the own vehicle changes lanes, and the first inter-vehicle distance is calculated by a first calculation unit. The second inter-vehicle distance is the distance required between the own vehicle and the forward/rear vehicles after the own vehicle or the forward/rear vehicles adjust their velocity, and the second inter-vehicle distance is calculated by a second calculation unit. A determination unit determines that the own vehicle can change lanes if the relative distance is equal to or greater than the required inter-vehicle distance.