1. An object detecting apparatus comprising: image acquiring means for acquiring both a present image and a background image photographed by a photographing unit, the background image being photographed when a detection target is not present in a monitoring area; background contour line information extracting means for extracting contour line information of the background image; and object detecting means for detecting whether or not a detection target is present in the monitoring area by judging whether or not contour lines other than contour lines of the background image can be extracted from the present image based upon the contour line information of the background image, wherein: when the object detecting means extracts a contour line at a pixel of a predetermined coordinate of the present image, the object detecting means judges whether or not the coordinate of the pixel corresponds to a coordinate at which a contour line is located in the background image based upon the contour line information of the background image; and if the object detecting means judges that the coordinate of the pixel corresponds to the coordinate at which a contour line is located in the background image, the object detecting means lowers a contour line detection sensitivity, and extracts the contour line at the pixel after the contour line detection sensitivity has been lowered.