An optical device to be connected to an optical fiber and an optical system in which the optical fiber is fixed to the optical device are provided, which optical device and optical system can restrict variations in a distance between inclined surfaces for supporting the optical fiber, and can enhance a productivity of the optical device and system. The optical system according to the present invention is, for example an optical multiplexer/demultiplexer having an optical device according to the present invention, which device has a substrate (2) with a crystal axis, and a groove (10a) for supporting an optical fiber (6a). The groove (10a) has a pair of opposing inclined surfaces (24) for supporting the optical fiber (6a), and a recess (26) formed between the pair of opposing inclined surfaces (24).