PURPOSE: To obtain an easy and economical directional coupler for single mode optical fibers having nearly truly circular core sections, by juxtaposing two pieces of eccentric optical fibers which are separated from each other at both ends and plural pieces of supporting optical fibers, melt-sticking thereof along the axial direction.

CONSTITUTION: Two pieces of eccentric optical fibers 5 which are separated from each other at both ends and form ports 11W14 for branching and two pieces of supporting optical fibers 15 are juxtaposed in the way that the side faces of the respective fibers are in contact with each other. These fibers are melt-stuck along their axial direction. The outside shape of a coupling part 8 is made circular by surface tension. Since the supporting optical fibers 15 are provided and are melt-stuck to the eccentric fibers, core parts 6 maintain the sectional shape which is nearly truly circular, whereby the easy and economical directional coupler for single mode optical fibers is obtained.