Touching devices have become one of the major elements in today's most electronic devices. As the increasing demands of large touching area, the state-of-the-art touching approaches become costly and infeasible. Therefore, it is essential to design a new kind of touch techniques with high touching accuracy and scalability with touching panel size. The purpose of this paper is to provide a touch system that uses the distance between the object (finger or stylus) and its shadow to detect the touch-timing and position. It can be applied to an interactive projection system without using large amount of touch-detecting elements. The proposed touch system only requires a camera and an IR source with an interactive projector to detect the occurrence of touching and its location. The proposed system achieves an average detection rate of 97.53% when the error tolerance is 10 pixels.