An optical device includes an optical base, an electric board, a fiber tray, and the like, which are placed in an enclosure. In particular, the electric board is made smaller than the optical base, and the fiber tray is placed in a space defined between an outer peripheral surface of the electric board and an inner side surface of the enclosure. In other word, the optical device uses a gap between the electric board and the enclosure not as a dead space but as a space for routing a remaining portion of the optical fiber. Thus, the optical device can improve space efficiency and thereby can reduce its size.