PURPOSE: To obtain a homogeneous waveguide structure composed of a core and a clad by forming a base material with clad and core members which are different from each other in refractive index and by heating and pressing the base material to manufacture a fiber.

CONSTITUTION: A substance with a higher refractive index such as TeBr or KRS-5 (thallium bromoidide) is used as a core member, and a substance with a lower refractive index such as TeCl or KRS-6 (thallium chlorobromide) is used as a clad member. The clad member 211 is formed into a cylindrical shape, and the core member 212 is put in the hollow of the member 211. A prescribed gap is left between both the members, and an intermediate member 213 is filled into the gap. The filler is made of AgCl, AgBr or the like with high transmittance to infrared rays including gaseous CO2 laser light with about 10.6μm wavelength. The resulting single crystalline base material is packed into a cylinder for extrusion and heated with a heating furnace, and by applying a pressure by means of a piston, a fiber having a waveguide structure composed chiefly of a core and a clad is obtd.