A cryptographic key distribution method, in which coherent light being suitable
for optical fiber communication network is used and high security is secured, is
provided. A sending end encodes random numbers so that symmetry probability distributions
can be obtained at a receiving end, and also sets light intensity and a modulation
index of signal light radiating from the sending end so that the SNR of an eavesdropper
is less than 2 dB even when said eavesdropper uses a most suitable receiving equipment
at the sending end, and also so that the SNR of the receiving end is more than
-10 dB, and transmits signals. The receiving end calculates probability distributions
of obtained signals and sets a discrimination threshold value after a set of random
numbers was transmitted from the sending end. When the probability distributions
have some abnormal states, it is judged that the eavesdropper exists, and distributing
the cryptographic key is stopped and a fresh cryptographic key is distributed again.
