An optical fiber coupling part that reduces the difficulty in adjusting cores, minimizes coupling loss, and prevents optical characteristics from deteriorating. An optical fiber is fused to one end of a GRIN lens which includes a quartz glass containing one or more refractive index regulating substances selected from Sb.sub.2O.sub.3, Ta.sub.2O.sub.5, TiO.sub.2 or ZrO.sub.2. Optical characteristics are not deteriorated because an adhesive is not used, and self-alignment effects facilitate adjustment of the cores of the GRIN lens and the optical fiber. Furthemore, coupling loss can be minimized by setting a refractive index distribution constant g of the GRIN lens within an appropriate range.