1. A vehicle rear view monitoring device including a plurality of distance sensors each for detecting a distance to an obstacle, a threshold level changing unit for changing a detection threshold level of each of said distance sensors to change a horizontal detection area. a vertical detection area, and a bearing resolution, an obstacle determination unit for determining whether or not an obstacle is existing according to a threshold level outputted from said threshold level changing unit and received signals of said distance sensors, and a display screen switching unit for changing a display image between a directly-captured image and a bird's-eye view image according to the distance to the obstacle determined by said obstacle determination unit, wherein said vehicle rear view monitoring device comprises: an overlay unit for measuring a bearing of the obstacle and a distance to the obstacle from a position where each of said distance sensors is mounted and the distance to the obstacle, and amplitude information about an amplitude of a reflected wave from the obstacle, the position and the distance to the obstacle being determined by said obstacle determination unit, for determining a display position and a shape of an obstacle mark according to a corresponding distance and a corresponding bearing on a screen for changing a display image as an expression of the obstacle mark between a directly-captured image and a bird's-eye view image when the distance to the obstacle determined by said obstacle determination unit has a predetermined value, and for overlaying the obstacle mark having said determined shape on an image captured by a camera and furnished from said display screen switching unit in such a way that the obstacle mark is positioned at said determined display position.