An optical fiber coupler is comprised of a fusion-elongated portion 15 in which an optical fiber 11 which is designed and fabricated for use at a wavelength in the vicinity of 1.55 .mu.m and an optical fiber 12 which is designed and fabricated for use at a wavelength in the vicinity of 0.98 .mu.m are fused and elongated. The propagation constant difference between optical fibers 11 and 12 is 10.sup.-4 rad/.mu.m or smaller.