Regardless of a transmission basis, a phase of a signal light with reference to a phase of local light is measured, and an output light from an antisqueezed light source in a transponder device is modulated in accordance with the measured phase. Since information obtainable in the transponder device is only the relative phase of the signal light with reference to the phase of the local light, and includes fluctuations corresponding to an antisqueezed component of the signal light, even if the information in the transponder device is eavesdropped on, the information cannot be decoded easily.