A first inter-vehicle distance serving as an indicator for a lane change by a home vehicle into an adjacent lane adjacent to the lane of the home vehicle is used to decide whether lane change by the home vehicle is enabled or disabled. In the event of a decision that lane change is disabled, a second inter-vehicle distance to the front/back of the first inter-vehicle distance is used to decide as to the possibility that the first inter-vehicle distance will expand to a length such that lane change is possible. In the event of a decision of such a possibility, it is decided that a wait is required, whereas in the event of a decision that there is no such possibility, it is decided that a wait is not required.