An optical branching device comprises a first lens member comprising a transmitting plane surface, a transmitting convex surface having an approximate center of curvature in the transmitting plane surface, and a first optical fiber and a second optical fiber firmly affixed on the transmitting plane surface of the first lens member. The transmitting convex surface has a wavelength-selective mirror surface that transmits light of a first wavelength and reflects light of a second wavelength. The light of the second wavelength, which is one of the wavelengths of light emitted from the first optical fiber and is reflected and converged by the wavelength-selective mirror surface, enters into the second optical fiber.