An ONU communicates with an OLT that can transmit optical signals having different wavelengths simultaneously and receive optical signals having different wavelengths simultaneously. The ONU includes: an optical transceiver that receives any one of the optical signals that the OLT can transmit and transmits any one of the optical signals that the OLT can receive; a communication failure detection unit that detects a communication failure between the OLT and the ONU; and a wavelength selection unit that, when the communication failure detection unit detects a communication failure, changes a setting of a downstream wavelength to be received by the optical transceiver and an upstream wavelength to be transmitted by the optical transceiver.