It is possible to establish a communication between an OLT and an ONU in an access network using an optical switch without requiring photoelectrical conversion by an optical switch unit. The OLT successively selects an ONU in slots discretely arranged by controlling a downlink optical switch SW(DOWN) and transmits a Discovery Gate message. The ONU which has received the Discovery Gate continuously transmits a Register Request message. An uplink optical switch SW(UP) successively switches signals from ONU#1 to ONU#128 from one to another in the slots discretely arranged and outputs the signalsto the OLT. Some of the Register Requests transmitted from the respective ONUs reach the OLT through the SW(UP). The OLT decides the ONU transmission timing in accordance with the received Register Request and reports the timing by a Gate message.