Human counting in cinema is easily influenced by varied illumination, so as to become a complicated problem. This paper develops an audience counting system in cinema by detecting occupied chairs in captured images. Firstly, we initialize chair regions in a background image manually. Then, the differences between the background and current images are detected as foreground regions. Such rough segmentation results always contain noise because of environmental illumination changing. Thus, a contour difference detection algorithm is applied to refine the audience detection results. Next, if both foreground and contour differences in a chair region are larger than a threshold, this chair is recognized to be occupied by an audience. Finally, the audience number is estimated by counting the occupied chairs.