A control ECU (10) executes left-side preliminary control for preventing an occupant from performing an operation of opening a left door when an object is predicted to reach a left virtual line segment (LVL) based on a position of the object. The control ECU executes right-side preliminary control for preventing the occupant from performing an operation of opening a right door when the object is predicted to reach a right virtual line segment (RVL) based on the position of the object. Further, the control ECU executes both-side preliminary control for preventing the occupant from performing an operation of opening both of the left door and the right door when the object is predicted to reach a central virtual line segment (CVL).