PURPOSE: To easily form an optical coupler having an excellent coupling efficiency by a simple process, by constituting an optical system of two lenses of two groups and specifying the media and shapes of both lenses.

CONSTITUTION: An optical system 2 is constituted of two groups of a 1st lens 5 and a 2nd lens 6 and the media of the lenses 5 and 6 are specified in such a way that the refractive index n(r) at a point which is apart from the optical axis by a distance (r) can satisfy an equation I. The n0 in the equation is the refractive index on the central axis and (g), h4, h6, and h8 are distribution constants. Moreover, one surface of the 1st lens 5 is a convex surface and the other is a plane perpendicular to the optical axis 7. The 2nd lens 6 has planes perpendicular to the optical axis 7 on both sides and is closely brought into contact with an optical transmission fiber 3. Therefore, the residual wavefront aberration becomes extremely small and the permissible value to the axis discrepancy in the direction vertical to the optical axis 7 becomes larger. Accordingly, an optical coupler having an excellent coupling efficiency can be formed by a simple process.