PURPOSE: To use a laser oscillator of small output as a lens having an excellent condensing characteristic and to improve laser beam machine by forming a convex face to the hyperboloid of revolution which rotates the hyperbola of the eccentricity coinciding with the value of the refractive index of a lens material with respect to the wavelength of laser light and providing a parallel plane part to the outer circumferential part.

CONSTITUTION: The convex face of the lens 1 is formed to the hyperboloid of revolution 2 which rotates the hyperbola of the eccentricity coinciding with the value of the refractive index of the lens material with respect to the wavelength of the laser light to be condensed. The other face is constituted as a flat face 3 and a collar 4 of a parallel plane is provided on the outer circumferential part of the lens. There are no spherical aberrations of the incident light on the lens in parallel therewith and, therefore, the condensing characteristic is improved if the eccentricity is so determined as to coincide with the refractive index of the lens material with respect to wavelength of the laser light. Since the lens is mounted by nipping the parallel plane part 4 on the plane part of a lens holder of the laser beam machine, the lens is surely mounted always pependicularly on the central axis of the holder and, therefore, the generation of comatic aberrations by the inclination of the lens is prevented. The extremely effective lens when used as the condenser lens for an IR laser beam machine is thereby obtd.