Provided is an inexpensive and compact optical transmission module having high coupling efficiency between an optical fiber and a light projecting element and/or a light receiving element. This optical transmission module includes a lead frame including an electric wiring pattern formed therein, a resin housing formed through insert-molding of the lead frame, and an electric device mounted on the lead frame and including a light projecting element for photoelectric conversion. The lead frame forms a slit positioning an optical fiber to be coupled and a reflection mirror reflecting and condensing light from the light projecting element to the optical fiber.