A fault localization apparatus for an optical line in a wavelength division multiplexed passive optical network is disclosed. The apparatus can detect light loss and fault positions as a channel exhibiting an abnormal receiving state of signals received by each of the upstream receivers is checked, and a monitoring optical signal is used, in which the monitoring optical signal is generated as a pulse is inputted to a light source of the downstream transmitter corresponding to the channel showing such an abnormal receiving state. The apparatus according to the present invention can detect fault positions of optical lines between the central office and the remote nodes and between the remote nodes and the optical network units, since the light source of downstream channel in which faults occur is used as a monitoring light source. The apparatus according to the present invention can be cost-effectively implemented.