An optical network system including an upper network equipment, a lower network equipment, and a plurality of transfer network equipments (200-1, 200-2) connecting the upper network equipment and the lower network equipment through independent optical links, wherein each of the plurality of transfer network equipments (200-1, 200-2) includes: an upper link optical interface device (210-1, 210-2) configured to perform transmission and reception of an optical signal to and from the upper network equipment and a lower link optical interface device (220-1, 220-2) configured to perform transmission and reception of an optical signal to and from the lower network equipment; an upper link monitoring unit configured to monitor whether an upper link failure occurs based on at least one of information of physical optical loss of a reception signal and information of whether a synchronization signal for maintaining system synchronization between network equipments is received, wherein the reception signal and the synchronization signal are received by an optical receiver of the upper link optical interface device (210-1, 210-2), the upper link failure including a failure occurring in at least one of an optical link used as a working line with the upper network equipment and an optical interface device associated with the optical link as the working line in the upper network equipment; and a packet control unit configured to perform control such that a report signal associated with the upper link failure is transmitted to the lower network equipment based on a monitoring result of the upper link monitoring unit, and wherein the lower network equipment includes a line control unit configured to switch a transmission and reception link of the optical signal to an optical link of a standby line when the report signal associated with the upper link failure is received from the transfer network equipment (200-1, 200-2) associated with the working line, characterised in that the upper link monitoring unit is further configured to monitor whether a logical failure occurs in the transfer network equipment (200-1, 200-2) itself based on the reception signal, wherein the packet control unit is configured to stop transmission and reception of the optical signal associated with a service packet to and from the upper network equipment through the upper link optical interface device (210-1, 210-2) when the logical failure occurs, wherein the logical failure includes a looping failure in the transfer network equipment (200-1, 200-2) itself, and wherein the upper link monitoring unit is configured to monitor whether the looping failure occurs based on whether a source is identical to a destination in a MAC address of the reception signal. an upper link optical interface device (210-1, 210-2) configured to perform transmission and reception of an optical signal to and from the upper network equipment and a lower link optical interface device (220-1, 220-2) configured to perform transmission and reception of an optical signal to and from the lower network equipment; an upper link monitoring unit configured to monitor whether an upper link failure occurs based on at least one of information of physical optical loss of a reception signal and information of whether a synchronization signal for maintaining system synchronization between network equipments is received, wherein the reception signal and the synchronization signal are received by an optical receiver of the upper link optical interface device (210-1, 210-2), the upper link failure including a failure occurring in at least one of an optical link used as a working line with the upper network equipment and an optical interface device associated with the optical link as the working line in the upper network equipment; and a packet control unit configured to perform control such that a report signal associated with the upper link failure is transmitted to the lower network equipment based on a monitoring result of the upper link monitoring unit, and wherein the lower network equipment includes a line control unit configured to switch a transmission and reception link of the optical signal to an optical link of a standby line when the report signal associated with the upper link failure is received from the transfer network equipment (200-1, 200-2) associated with the working line, characterised in that the upper link monitoring unit is further configured to monitor whether a logical failure occurs in the transfer network equipment (200-1, 200-2) itself based on the reception signal, wherein the packet control unit is configured to stop transmission and reception of the optical signal associated with a service packet to and from the upper network equipment through the upper link optical interface device (210-1, 210-2) when the logical failure occurs, wherein the logical failure includes a looping failure in the transfer network equipment (200-1, 200-2) itself, and wherein the upper link monitoring unit is configured to monitor whether the looping failure occurs based on whether a source is identical to a destination in a MAC address of the reception signal.