A sender transmits to a receiver an optical signal that is phase-modulated in accordance with source data and a basis stored in a memory. The receiver phase-modulates the received optical signal in accordance with a basis, obtains detection data through interference, and stores the detection data in a memory. An inter-device address difference (GD) and an intra-device address difference (DI) are provisionally set. The detection data are checked against the source data while sequentially changing the values of GD and DI within a predetermined adjustment range. Based on the result of this checking, GD and DI are determined.