For conventional cipher communications based on the principles of quantum mechanics, the photon number per signal needs to be controlled below 1 or to a mesoscopic level in order to make quantum-mechanical properties remarkable. This invention is intended to provide a quantum-mechanical communications' method that is practical even with a macroscopic number of photons. Antisqueezed light is transmitted using a random sender basis. The legitimate recipient can use a secret key to know the random sender basis, and thus to receive the information accurately without being affected by antisqueezing. However, because eavesdroppers unknowing of the sender basis must use haphazard bases in an attempt to receive the signals, the eavesdroppers are significantly affected by antisqueezing and the respective signal bit-error-rates are increased. This makes eavesdropping impossible, thus ensuring secure communications between the legitimate senders and recipients.