An optical device (1) includes: a first capillary tube assembly (2) and a second capillary tube assembly (3), in which capillary tubes (6, 10) into which optical fibers (5, 8 (9)) are inserted and fixed, are retained, respectively, by retaining sleeves (7, 11); and a lens assembly (4) including a first lens (12) and a second lens (13), which are opposed to leading ends (5a, 8a (9a)) of the optical fibers (5, 8 (9)), respectively, through a space; a band pass filter (14) interposed between both lenses (12, 13); and an accommodation member (15) for accommodating those components. At least one of the lens (12) is formed of a spherical portion (12b) and a flat surface portion (12a), the flat surface portion (12a) is formed into an inclined surface, and a center of curvature of the spherical portion (12b) is offset from the optical axis of the optical fiber (5).