Disclosed herein is an apparatus and method for detecting an optical line fault in a Passive Optical Network (PON). The apparatus includes an optical distribution unit configured to, when a multiplexed signal of a downstream light signal and a monitoring light signal is input, distribute the multiplexed signal to an Optical Network Unit (ONU) of a first optical path and to a second optical path. An optical layer management unit is installed in the second optical path and is configured to set a time at which the monitoring light signal of the multiplexed signal provided to the second optical path is received to a starting time of monitoring light measurement, compare a monitoring light signal reflected and returned from the ONU with a signal pattern obtained when no fault occurs, and then determine whether a fault has occurred in a distribution network.