An optical device includes an optical fiber having a core and a cladding, a slit extending from the upper surface of the optical fiber into a glass substrate, a filter member inserted in the slit, and a PD array for detecting divided light. A first high-polymer gel material fills a gap between the slit and a filter member within the slit. A second high-polymer gel material is interposed between an optical fiber array and the PD array and disposed on an optical path of the light divided by the filter member.