A large diameter fiber is composed of a multimode optical fiber and provided with a fiber body having a constant diameter in an optical axis direction XA and a tapered section tapered in diameter toward a light exit surface. An adhesive member attaches the large diameter fiber inside a retaining hole of a tubular housing such that an outer circumferential surface of a tapered clad of the tapered section is entirely exposed to air to a predetermined depth from the light exit surface. A light passing space is a ring-like space formed between the exposed outer circumferential surface of the tapered clad and an inner circumferential surface of the tubular housing. Light in the tapered section is output from the light exit surface and partially leaked to the tapered clad. A part of the leaked light is released from the light passing space.
