To reduce transmission loss generated when optically connecting an optical fiber with a large emission area and an optical fiber or a light- receiving semiconductor with a small incidence area.

An optical fiber and an optical fiber, or an optical fiber and a light-receiving semiconductor are connected by interposing a multitube- or multilayer-shaped light guide. The light guide has a shape tapering off from a large optical fiber toward a small optical fiber or the light-receiving semiconductor. The refractive index of an optical transmission body is preferably nearly equivalent to the refractive index of the optical fiber. A flexible light transmission body is preferably mounted on one side or both sides of the connection end surface of a light guide. About characteristics of the light transmission body, its refractive index is preferably nearly equivalent to that of the optical fiber, and its hardness is preferably 50° or less in JIS (A type).