PURPOSE: To manufacture the titled lens capable of bonding many optical fibers efficiently to other optical fibers, by diffusing a substance capable of increasing the refractive index thermally into a heated part of a substrate heated locally by irradiation with laser beams.

CONSTITUTION: A surface for forming a microlens on a quarts or silicon substrate 3 is wrapped with a gas 2 of a substance, e.g. GeCl4, of about several tens of nm thickness capable of increasing the refractive index. The substrate 3 is then irradiated perpendicularly with laser beams 1 having controlled spot size and power and locally heated to about the melting point thereof to diffuse the substance, e.g. Ge, pyrolyzed from the gas 2 thermally into the surface of the substrate 3 and form a plate microlens having a refractive index distribution 5 proportional to the temperature distribution 4.