An optical masking apparatus of an OTDR capable of lowering insertion loss and of easily aligning optical axes and of being structured at low cost. The apparatus comprises an optical pulse generating means, an optical branching coupler for permitting an optical pulse generated by the optical pulse generating means to be incident to an optical fiber to be measured, and an optical chopper means to which a reflected light from the optical fiber is incident by way of the optical branching coupler wherein the optical chopper means operates in synchronization with the optical pulse to remove a desired range of the reflected light. A light signal supplied to an optical/electric transducer from the optical chopper means is converted into an electric signal, which is displayed on an indicator by way of a signal processing circuit.