This paper presents an efficient ray-launching method for 2D indoor propagation problem, by including crucial multiple reflection effect inside wall. We here focus on the derivation of the reflected and transmitted ray solutions at/through wall when a magnetic source is located in the indoor environment. An efficient approximation, which is called collective ray approximation, is carried out to bundle or collect the internal multiple reflected rays into the primary one. It is resultantly confirmed from the detailed numerical experiments that the derived collective ray solutions can be confidently effective when the internal reflections strongly contribute to the propagation characteristic in the propagation environment, regardless of angle of the incidence.