An input port and an output port are provided to a wave transmission medium. Field distribution 1 of a propagated light (forward propagated light) that has entered through the input port is determined by numerical calculation. Field distribution 2 of a phase conjugate light (reversely propagated light) is also determined by numerical calculation. The phase conjugate light is such that the output field expected when an optical signal that has entered through the input port is outputted from the output port is propagated reversely from the output port. From field distributions 1, 2, the spatial distribution of the index of refraction in a medium can be so determined that there is no phase difference between the propagated light and the reversely propagated light at each point (x, z).