A stress in an optical fiber due to a deformation of the optical fiber is surely alleviated without damaging the optical fiber. For this purpose, the optical fiber coated with a coating material is treated with heat for a short time (about 0.1 seconds to 30 seconds) at a temperature higher than the glass transition temperature of the glass of which the optical fiber is composed. Otherwise, the optical fiber coated with the coating and bent into an arbitrary shape such as a circle is uniformly irradiated with a light having a photon energy near the edge of the absorption attending upon an electronic transition of the glass of which the optical fiber is composed.