PURPOSE: To provide an illuminator which can be manufactured at a moderate cost, can be handled easily, and can assure the effective utilization of light emitted from a light source.

CONSTITUTION: An illuminator is provided with a light source 10 and an optical core light guide 16 having a tubular flexible sheath made of a fluorocarbon polymer. The guide 16 is filled up with glycol; a salt solution containing a chloride, fluoride, or phosphate; phenylntethyl silicone oil (AN 140, n=1.57); PCTFE oil; poly(chlorotrifluroethylene) oil (n = about 1.40); or pure water. The internal surface of the sheath 21 is coated with a thin film of Teflon AF(R) having a refractive index smaller than that of the material used for making the sheath.