Apparatus and method for laser machining

A laser machining method using a laser machining apparatus, said apparatus including a laser oscillating section for oscillating a laser beam;

• a laser machining head;
• an optical fiber, including a core and a clad disposed around said clad, for transmitting said laser beam oscillated by the laser oscillating section into the laser machining head, said optical fiber cooperating with the laser machining head to form a beam transmitting section;
• and an assist gas supply for supplying an assist gas of oxygen to the laser machining head;
• wherein said laser beam is transmitted through said optical fiber and projected against a work to cut the work while supplying said assist gas to a cutting point of said work,
• the method comprising:
• removing or reducing said laser beam transmitting in said clad of said optical fiber or said laser beam projected from the clad so that an energy density of said laser beam transmitted from said clad and measured on said work is 15 kW/cm2 or less.