Provided is an optical device in which connection loss is reduced while practicality is enhanced. A multi-core fiber connection device (100) is an optical device for connecting a multi-core fiber (10) to a single-core fiber (20), and comprises a first optical system (S1) positioned on the optical axes of a plurality of beams emitted from a multi-core fiber (10), the first optical system for separating the optical axes of the beams from each other by making the optical axes non-parallel to each other, and a second optical system (S2) for making the optical axes of the plurality of beams substantially parallel to each other, the optical axes having been non-parallel to each other at the first optical system (S1).