An ECU calculates: a free running distance, which is a distance that a vehicle can travel from a first time point at which a speed-reduction control is started to a second time point at which the relative deceleration begins to increase by the start of the speed-reduction control; an increase travel distance, which is a distance that the vehicle can travel from the second time point to a third time point at which the relative deceleration reaches the target relative deceleration; and a post-completion travel distance, which is a distance that the vehicle can travel from the third time point to a time point at which the relative speed is made equal to or less than the specified speed. The ECU obtains a speed reduction distance based on a result of adding up the free running distance, the increase travel distance, and the post-completion travel distance.