To provide an optical part for laser beam emission in which the energy utilization efficiency is improved by making use of the light reflected from the object to be machined.

A laser beam is emitted from a laser oscillator 10. The laser beam emitted from the laser oscillator 10 is converged on the spot to be machined on the surface of an object 32 to be machined by a condenser lens 24. The light reflected from the surface of the object 32 to be machined is reflected by a concave mirror 25 toward the surface of the object 32 to be machined. A reflection mirror is moved relatively to the condenser lens 24 in the direction parallel with the optical axis of the condenser lens 24 by fixing mechanism of the reflection mirror. The position relative to the condenser lens 24 is fixed at a certain position.