The light beam generator 1 is provided with a laser light source 10, an optical phase modulation element 15 and others. The optical phase modulation element 15 receives coherent light output from the laser light source 10 and passed through a beam splitter 14 to modulate a phase of the light depending on a position on the beam cross section of the light, and outputs the light after the phase modulation to the beam splitter 14. A polar coordinate system (r, θ) in which a predetermined position is given as an origin is set on the beam cross section of the light input in the optical phase modulation element 15, and when (p+1) domains divided by p (number of pieces) circumferences in which the predetermined position is given as a center are set, of these (p+1) domains, a phase modulation amount φ at each position inside an even numbered domain counted from the inside is expressed by a formula of “φ=qθ”, and a phase modulation amount φ at each position inside an odd numbered domain counted from the inside is expressed by a formula of “φ=qθ+π”.