An optical device guides propagating light to exit from a desired side surface of a substrate while maintaining sufficient power within a limited size of the substrate and which facilitates adjustment of a mount position of a light-receiving element by increasing allowable tolerance of the mount position of the light-receiving element. The optical device has a substrate, a light propagating direction path formed in the substrate, and a reflection groove formed in an output-side of said light propagating direction path in the substrate. A side wall of the reflection groove is formed to become a convex reflection surface in relation to light which enters from the light propagating direction path, and the light output from the light propagating direction path undergoes total reflection on a side wall forming the reflection surface and lets the reflected light exit from a predetermined side surface of the substrate while enlarging an image.