A light source-optical fiber coupler using a gradient index rod lens 12 by which a diffused luminous flux emitted from a light source (for example, a semiconductor laser 10) is coupled onto an end surface of an optical fiber (for example, a single-mode optical fiber 14). The gradient index rod lens has a planar end surface facing the light source, and a convex spherical end surface facing the optical fiber. The gradient index rod lens has a light source side numerical aperture NA2 in a range of from 0.40 to 0.75, an effective lens radius r0 in a range of from 0.3 to 1.0 mm, and a spherical curvature radius R1 in a range of from 1.2 to 2.0 mm.