A laser processing machine comprising: a first lens that is movable in an optical axis direction and having a positive focal length, and configured to convert divergent light of a laser beam emitted from an emission end of the laser beam into convergent light; a second lens that is movable in the optical axis direction and having a negative focal length on which the convergent light is incident; a third lens having a positive focal length, configured to focus the laser beam emitted from the second lens, and to irradiate a plate material with the focused laser beam; a first moving mechanism for moving the first lens; a second moving mechanism for moving the second lens; a first driver configured to drive the first moving mechanism; a second driver configured to drive the second moving mechanism; and a controller configured to control the first and second drivers (282 and 302) so as to move the first and second lenses (28 and 30), respectively, wherein the controller is configured to control the second driver so as to dispose the second lens at a position, which is shifted from a position where the convergent light is focused, toward the first lens side by the same distance as the focal length of the second lens according to a position of the first lens in the optical axis direction, so as to convert the convergent light into parallel light, wherein the controller is configured to control the first driver so as to dispose the first lens at a position where a distance from the emission end to the first lens is equal to or greater than a focal length of the first lens, wherein a focused beam diameter, that is a diameter of a beam waist of a beam irradiated on the plate material, is represented by d, a beam parameter product, that is a product of a radius d/2 of the beam waist and a half width at half maximum of a divergence angle of the beam irradiated on the plate material, does not change, and is a fixed value, even when the first and second lenses (28 and 30) are moved, wherein when a beam diameter of a parallel light emitted from the second lens is represented by D, the beam parameter product is represented by BPP, and a focal length of the third lens is represented by f, the controller is configured to control the first and second drivers (282 and 302) so as to set relative positions of the first and second lenses (28 and 30) in such a way that the beam diameter D is increased so as to decrease the focused beam diameter d as a plate thickness of the plate material decreases, and that the beam diameter D is decreased so as to increase the focused beam diameter d as the plate thickness of the plate material increases, and the controller is configured to determine the focused beam diameter d based on Equation. d = 4 ⋅ BPP ⋅ f D a first lens that is movable in an optical axis direction and having a positive focal length, and configured to convert divergent light of a laser beam emitted from an emission end of the laser beam into convergent light; a second lens that is movable in the optical axis direction and having a negative focal length on which the convergent light is incident; a third lens having a positive focal length, configured to focus the laser beam emitted from the second lens, and to irradiate a plate material with the focused laser beam; a first moving mechanism for moving the first lens; a second moving mechanism for moving the second lens; a first driver configured to drive the first moving mechanism; a second driver configured to drive the second moving mechanism; and a controller configured to control the first and second drivers (282 and 302) so as to move the first and second lenses (28 and 30), respectively, wherein the controller is configured to control the second driver so as to dispose the second lens at a position, which is shifted from a position where the convergent light is focused, toward the first lens side by the same distance as the focal length of the second lens according to a position of the first lens in the optical axis direction, so as to convert the convergent light into parallel light, wherein the controller is configured to control the first driver so as to dispose the first lens at a position where a distance from the emission end to the first lens is equal to or greater than a focal length of the first lens, wherein a focused beam diameter, that is a diameter of a beam waist of a beam irradiated on the plate material, is represented by d, a beam parameter product, that is a product of a radius d/2 of the beam waist and a half width at half maximum of a divergence angle of the beam irradiated on the plate material, does not change, and is a fixed value, even when the first and second lenses (28 and 30) are moved, wherein when a beam diameter of a parallel light emitted from the second lens is represented by D, the beam parameter product is represented by BPP, and a focal length of the third lens is represented by f, the controller is configured to control the first and second drivers (282 and 302) so as to set relative positions of the first and second lenses (28 and 30) in such a way that the beam diameter D is increased so as to decrease the focused beam diameter d as a plate thickness of the plate material decreases, and that the beam diameter D is decreased so as to increase the focused beam diameter d as the plate thickness of the plate material increases, and the controller is configured to determine the focused beam diameter d based on Equation. d = 4 ⋅ BPP ⋅ f D