The converging li itt guide employed in the present invention may be analogized to the gas lens. (See, for example, the Proceedings of the IEEE, Dec. 1965, pages 2148-2l49, wherein S. Kawakami and .I. Nishizawa describe such a light guide as similar to a lens.) When a laser beam of fundamental mode is made incident upon the input end surface of the light guide, the light beam should have a specifi