PURPOSE: To obtain an optical fiber coupler with which the coupling loss of light is virtually eliminated or extremely slight and of which the system is not intricate by polishing diagonally the clad layer of an optical fiber most widely in the end face part thereof.

CONSTITUTION: When the clad layer 2 of an optical fiber is diagonally polished at a polishing angle θ so as to make the cutting area widest at the end face part thereof, the NFP shown in the figure is obtd. The figure reveals that the NEPd at the segment D-D is symmetrical as the clad layer in said part is not diagonally polished at all and that the degree of asymmetry increases gradually with an increase in the degree of the diagonal polishing, i.e., with a change to the segment C-C, segment B-B and segment A-A. The optical fiber can be cut perpendicularly to the optical axis at the optional point indicating the NFP matching the NFP of the optical waveguide to be coupled while the NFP at the end face is measured after completion of the diagonal polishing in the above- mentioned way. The coupling loss is made nearly zero or suppressed to nearly zero with the optical fiber coupler shown in the figure in which the optical fiber after cutting is couuled to the optical waveguide.