In this paper, we propose a real-time people counting system with a single camera for security inside the building. The camera is hung from the ceiling of the door so that the image data of the passing people are not fully overlapped. The implemented system recognizes people movement along various directions. To track people even when their images are partially overlapped, the proposed system analyzes the captured image and estimates important feature information. Then each bounding box enclosing each person in the tracking region are tracked. The convex hull of each individual obtained by the proposed algorithm provides more accurate tracking information. Through extensive tests in various environments, the proposed system exhibits the correct people counting results of over 96%.