To prevent enlarged recognition deviation between a station-side device and a customer-side device during a certain period in a sleep mode.

The customer-side device (ONU) includes the sleep mode and a non-sleep mode. In the sleep mode, the customer-side device (ONU) generates a period Tb, in which communication between the customer-side device (ONU) and a station-side device (OLT) is suspended, and a period Ta, in which communication between the customer-side device (ONU) and the station-side device (OLT) is possible. In a normal mode of the customer-side device (ONU), clock is synchronized between the customer-side device (ONU) and the station-side device (OLT). If a lapse period of the sleep mode reaches a set continuation period, the customer-side device (ONU) terminates the sleep mode and shifts to the normal mode.