The invention relates to a laser processing device that can change the diameter of a beam spot on an object to be processed, without using a mechanism for changing a distance between a condensing lens and the object. A beam emitted from a laser beam source is condensed by the condensing lens, and is made incident to an optical fiber. The beam emitted from the optical fiber is condensed by a condensing optical system onto an object to be processed. The object is welded using the condensed beam. The laser processing device comprises an adjusting unit that can change the angle of divergence of the beam on the emission surface of the optical fiber. The adjusting unit changes the angle of divergence of the beam, thereby to change the diameter of the beam spot formed on the surface of the object.