Laser light is emitted from a laser oscillator, and the laser light is made to enter a beam expander optical system including a concave lens through a correction lens. The laser oscillator, the correction lens and the concave lens are disposed so that, when an emission point of the laser oscillator is a first conjugate point, a point at which an image at the first conjugate point is formed through the correction lens is a second conjugate point, a distance between the correction lens and the second conjugate point is b, a focal length of the concave lens is f, and a distance between the correction lens and the concave lens is X, the X satisfies b-3|f|.ltoreq.X.ltoreq.b+|f|.