When either a front-side sensor or a rear-side sensor of a subject vehicle detects an obstacle entering a certain distance range of multiple distance ranges within a detection zone close to the subject vehicle, a micro-computer causes a warning unit to warn with a buzzer in a primary warning pattern that is prepared for each distance range. In contrast, when both detect obstacles, the warning unit is caused to warn in the following patterns. In a first case that at least one of two sensors detects an obstacle entering a shortest distance range, the warning unit warns in a different warning pattern differentiated from the primary warning pattern. In a second case except the first case, the warning unit warns in the primary warning pattern corresponding to the distance range that is entered by the obstacle that is closer to the vehicle, of the obstacles detected by the two sensors.