To provide a device for distributing/transmitting a single-photon quantum key which prevents 3 dB principle loss in a transmitter, provides a high transmission rate by eliminating a polarization compensation mechanism on a receiver side, and requires a simple structure.

In this device for distributing/transmitting a quantum key, phase modulation is individually applied to two double light pulses 7, 8 appearing in two output ports 5, 6 of an asymmetrical Mach-Zehnder interferometer 2 having a transmission side, and thereafter they are integrated with each other by a polarization beam splitter 12. By setting a relation of set values of two phase modulators to (1-2=π), the need of polarization compensation in reception is eliminated.