We propose a robust mesh-to-mesh collision detection algorithm using a ray tracing method. The algorithm checks all vertices of a geometrical object based on the proposed criteria, and then the colliding vertices are detected. In order to realize a real-time calculation, acceleration by spatial subdivision is performed. Since the proposed ray-traced collision detection method can directly calculate the reacting forces between the colliding objects, this method is apt for a real-time medical simulation dealing with deformable organs. Our method addresses the limitation of the previous ray-traced approach as it can detect collisions between all arbitrarily shaped objects, including non-convex or sharp objects. Moreover, deeply penetrated collisions can be detected effectively.