PURPOSE:To leak light transmitted from the side face of optical fiber to a wide range by gradually reducing the thickness of a clad layer in the optical fiber in an underwater part in the forward direction of the light. CONSTITUTION:The treating time with a mixed solvent of, e.g. acetone/methanol or the mixing ratio thereof is regulated in a clad treated part 5 of optical fiber made of a plastic in an underwater part where underwater organisms live or the mixing ratio is regulated so as to gradually reduce light transmitted in the interior of optical fiber in the longitudinal direction. The clad 9 on the incident side is kept in a thick state and the clad layer 9 is thinned toward the outlet side of the light transmitted with a core 8 and the clad 9 by this regulating treatment. Thereby, the three-dimensional irradiation, irradiation range and irradiation quantity can be changed to proper ones by one optical fiber.
