PURPOSE: To transmit only the light which is parallel along the axial core of a transmitter, by coating a completely absorptive black body like carbon, etc. over the circumferential surface of a linear core of glass, synthetic resin, etc.

CONSTITUTION: An adhesive 4 containing the powder of a completely absorptive black body 3 like carbon, etc. and having the same refractive index as a linear core 1 is coated over the circumferential surface of the core 1 made of glass or synthetic resin. Thus a cladding 2 is formed. In such constitution, the light A which is incident right-angled to the incident end face is emitted to the other end face as shown by a solid line. While the oblique line B goes into the adhesive 4 having the same refractive index and is absorbed to the powder carbon, etc. Thus the light B does not reach the other end face. When such transmitter is set in front of a photodetecting sensor, a telephoto lens, etc., the undesired light can be cut.