An object detecting apparatus, comprising: image acquiring means arranged to acquire 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 (2, 25) arranged to extract contour line information of the background image; and object detecting means (3, 18, 26, 35) arranged to detect whether or not a detection target is present in the monitoring area by judging whether or not contour lines other than the a 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 (3, 18) extracts a contour line at a pixel of a predetermined coordinate of the present image, the object detecting means (3, 18) is arranged to judge 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, if the object detecting means (3, 18) judges that the coordinate of the pixel does not correspond to the coordinate at which a contour line is located in the background image, the object detecting means (3, 18) is arranged to extract the contour line at the pixel by comparing a difference Δ(x, y), which comprises a difference between a weighted luminance of the pixel and a luminance of an adjacent pixel, with a first threshold, and if the object detecting means (3, 18) 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 (3, 18) is arranged to extract the contour line at the pixel by comparing the difference Δ(x, y) with a second threshold that is larger than the first threshold, such that contour line detection sensitivity is lowered. image acquiring means arranged to acquire 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 (2, 25) arranged to extract contour line information of the background image; and object detecting means (3, 18, 26, 35) arranged to detect whether or not a detection target is present in the monitoring area by judging whether or not contour lines other than the a 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 (3, 18) extracts a contour line at a pixel of a predetermined coordinate of the present image, the object detecting means (3, 18) is arranged to judge 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, if the object detecting means (3, 18) judges that the coordinate of the pixel does not correspond to the coordinate at which a contour line is located in the background image, the object detecting means (3, 18) is arranged to extract the contour line at the pixel by comparing a difference Δ(x, y), which comprises a difference between a weighted luminance of the pixel and a luminance of an adjacent pixel, with a first threshold, and if the object detecting means (3, 18) 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 (3, 18) is arranged to extract the contour line at the pixel by comparing the difference Δ(x, y) with a second threshold that is larger than the first threshold, such that contour line detection sensitivity is lowered.