An optical device includes a glass substrate provided with one or more V-shaped grooves and an optical fiber array having one or more optical fibers. The optical fiber array is fixed within the V-shaped grooves of the glass substrate using a fixing adhesive. As viewed from above, a first region Z1 (i.e., the region indicated by cross-hatching) and a second region Z2 are provided on the optical fiber array, and the fixing adhesive is applied within the first region Z1. A refractive index matching resin, providing refractive index matching at least with the optical fibers, is disposed in the second region Z2, wherein the second region Z2 may be surrounded by the first region Z1.