PURPOSE: To irradiate an optical fiber which is coupled at both ends to light sources linearly with constant brightness by forming flaws reaching the core thereof to said fiber and making the density thereof nonuniform.

CONSTITUTION: An optical fiber 2 is coupled at both ends to light sources 1. Many flaws 4 reaching the core from the surface are formed to the fiber 2, and the density of the flaws 4 is made nonuniform in the extension direction of the fiber 2. Since the photoelectric power injected into the fiber 2 from both ends decreases toward the central part, the density of the flaws 4 is increased toward the central part of the fiber 2 conversely from the attenuating direction of the photoelectric power. When the density of the flaws is defined as (n), the position in the extension direction of the optical fiber as γ, the length of the optical fiber to be illuminated as 1, and a constant as C, the flaws are formed in the respective positions of the optical fiber at n=1/Ce-γ density from γ=0 up to γ1/2, and n=1/Ce-(1-γ) density from γ=1 2 up to γ=1.