Disclosed herein are an optical network unit and a method for controlling the unit. In a passive optical network system, an optical transceiver of the optical network unit performs a wavelength change of an optical signal according to a wavelength setting command transferred from a media access control (MAC) unit and monitors a wavelength change state and reports the monitored wavelength change state to the MAC unit. When the wavelength malfunction occurs, the MAC unit blocks an uplink optical transmission of the optical transceiver.