An optical layer monitoring apparatus and method thereof are provided. According to an embodiment of the present invention, an optical layer monitoring apparatus including an optical time domain reflectometer (OTDR) function so as to monitor an optical path of a passive optical network (PON), and a method for improving accuracy of measured monitoring results using the optical layer monitoring apparatus are provided. Therefore, it is possible to enable a user to continuously detect distortion or attenuation along the optical path, and to quickly recover from the distortion or attenuation along the optical path when distortion or attenuation is detected.