To generate a Bessel beam with large depth of a focus by making the light beam, projected from a light source means emitting a light beam with a cylindrical light intensity distribution, incident on an image formation optical system.

The light source means 12 which emits the light beam (laser beam) with the cylindrical light intensity distribution consists of, for example, a vertical oscillation type surface light emission laser (semiconductor laser) and generates the laser beam (light output) 11 having the cylindrical (doughnut- shaped) light intensity distribution. Then a convex lens 22 as the image formation optical system is arranged at a distance of the focal length (-f) of the convex lens 22 from the laser end surface (light emission surface) of the vertical oscillation type surface light emission laser 12 and generates the Bessel, beam in an interference area (Bessel beam area) almost at a distance of the focal length (+f) of the convex lens 22. Thus, the Bessel beam having the intensity distribution which is nearly in proportion to the square of a 0-order Bessel function of 1st class is generated in the interference area (Bessel beam area) 21.