An optical device which reduces coupling loss while improving practicality is provided. A multi-core fiber coupling device is an optical device which couples a multi-core fiber to single core fibers, and includes a first optical system which is located on optical axes of a plurality of beams emitted from the multi-core fiber, and which makes the optical axes of the respective beams non-parallel to each other, thereby making the beams in a state of being separated from each other, and a second optical system S2 which makes the optical axes of the plurality of beams in a state of being non-parallel to each other on the side of the first optical system, in a state of being approximately parallel to each other.