In cellular systems, propagation prediction is important in determining cell area. There are two well known methods for predicting propagation loss. One uses propagation prediction formulas (empirical formulas) generated from measured field strength data, and the other method uses ray-tracing and so is based on geometrical optics (GO). In order to improve the propagation prediction accuracy of the ray-tracing method, multiple reflection and multiple diffraction must be considered. However, it is very difficult to calculate multiple reflection and diffraction losses precisely. Previous studies used simple approximate models to avoid this difficulty. In this paper, we propose a method for calculating multiple diffraction losses precisely, and show prediction results in the case where this method is applied to a propagation prediction system.