In an optical system in which light output from an optical fiber (11) is collimated into parallel light rays by a gradient index rod lens (13), reflected by a filter (4) and then converged again by the rod lens (13) so that the resulting light is coupled to an optical fiber (12), a principal beam component of the light output from the optical fiber (11) disagrees with an optical axis (32) of the optical fiber (12) to thereby cause a coupling loss. When W is the distance between optical axes (31 and 32) of optical fibers (11 and 12) and Z is the length of a rod lens (13) on an optical axis (33) of the rod lens, the optical fibers (11 and 12) and the rod lens (13) are disposed to satisfy the condition: W·g·(0.25−Z/P)2≦6×10−5 in which g is a gradient index distribution coefficient of the rod lens, and P is the periodic length of the rod lens (13).