A method of positioning reflecting mirrors inside a multi-level optical switch. The multi-level optical switch has a plurality of input terminals and a plurality of output terminals. A first fixed single-sided reflecting mirror and a second fixed single-sided reflecting mirror are installed such that the reflecting surfaces of the first and the second fixed single-sided reflecting mirrors face each other and are parallel to each other. The input terminals are divided into an odd group and an even group. Cross-points of optical paths are computed by feeding the input light paths from the odd group and the even group using a network exchange algorithm. Double-sided reflecting mirrors are positioned at the corresponding optical path cross-points in the double-sided reflecting mirror layout region of the multi-level optical switch.