In an optical transmission system, a lens converges an optical signal outputted from a light emission element. The optical signal having passed .through the lens enters a multi-mode fiber (MMF). A vertex of the lens and an input plane of the MMF are at a distance. The distance is set to a value which is not equal to the distance from the vertex to a focal point of the lens. As a result, a lowcost optical transmission system can be provided in which the influence of mode dispersion is reduced.