Wireless communication systems such as mobile phones and wireless LAN are affected by walls and windows when those systems are used inside/around buildings. For making clear the fact, electric field distribution inside/around the model of a square building are measured and analyzed by FVTD simulations taking into account the difference of windowpanes' structures. In this paper, to know the effect of the windows' thickness, electric field distribution around the building model with double-layered windows is compared with that with single-layered windows. Those results show that the electric field strength is made higher and is attenuated more by the double-layered windows compared with single layered windows inside and outside the building, respectively. Therefore, it has been shown that the EM propagation around double layered windows is different from that around single layered windows.