PURPOSE: To enlarge the working range of cutting, scribing, welding, etc., by providing a tubular internal mirror having an inlet of specific size at a focal position of a condenser lens and length for focusing energy to the center part in an outlet by utilizing the influence of an interference.

CONSTITUTION: The machine is provided with an internal mirror 16 of a straight pipe having an inlet 17 whose size is ≥ ten folds of wavelength of a laser L at a focal position of a condenser lens 11, and having the inlet 17, an outlet 18 and length which can focus energy to the center part of the outlet of the internal mirror by executing a calculation of an energy distribution in the outlet of the internal mirror by tracking of a light beam and a principle of superposition and utilizing the influence of an interference, and at the outlet 18, energy is focused to the center part. In this regard, when the same tubular internal mirror 19 as the foregoing in which as outlet 21 is smaller than an inlet 20 is provided at a focal position of the condenser lens 11, an increase of the energy density and a concentration of an energy distribution shape can be enhanced.