An optical loopback apparatus provides a basis for miniaturizing the loop back mechanism so as to enable optical attenuation and wavelength multi/demultiplexing operations to be carried out in a compact device. An optical fiber unit has two single core fibers for input and output fiber paths. By aligning the central axis between the cores with the optical axis of a lens having a refractive index distribution of 0.25 pitch, the input beam into the apparatus is wholly reflected back 180 degrees by a total reflection film provided at the output terminal surface of the lens. In another arrangement, the optical axis of all four components: the central axis of the input and output fiber paths; the first lens; a second lens and a second optical fiber; are aligned, then, an input beam into the first lens is partially looped back 180 degrees at the reflective film.