A testing apparatus is presented which evaluates the performance of optical signal transmissions in the 1.3 .mu.m wavelength region, independently or simultaneously with signal transmission. An optical signal is transmitted in the network.fwdarw.user direction in a thermo-optic (TO) switch 11, and the signal is also looped in the direction A of an optical directional coupler 3. An optical time domain reflectometer (OTDR) 10 Generates a test beam at 1.55 .mu.m wavelength region, which is reflected by a wavelength selective filter 8 back to the OTDR 10 in the user-network direction. The apparatus is thus able to perform testing of the transmission cable 7 simultaneously with the signal transmission. When an operating problem is to be pinpointed in the system, the optical switch 11 is switched so as to permit the signal from a line terminal LT 1 to transmit in the network.fwdarw.user direction. The signal is attenuated by the same magnitude as the loss budget between the terminals NT 9 and LT1, and is looped back to LT 1, thereby enabling Loop 1 testing at the same signal strength level as in the actual signal transmission.