An avoidance route calculation part calculates an avoidance route for avoiding a collision between an own vehicle and an obstacle through a collision avoidance assist control (an automatic brake control and an automatic steering control). A post-avoidance route calculation part calculates a post-avoidance route. A post-avoidance route collision determination part determinates whether a secondary obstacle is present on the post-avoidance route. When the secondary obstacle is determined to be present, the automatic steering control is prohibited from being performed.