Laser beam processing apparatus

This laser processing apparatus includes an optical fibre for amplification (amp fibre (10)), a seed laser oscillating unit (12), a fibre core exciting unit (14), a laser emitting unit (16), a controlling unit (20), and a light sensor (64). A Q-switched pulse seed laser beam from the seed oscillating unit (12) enters into one end surface of the amp fibre (10), and a continuously oscillated core excitation light from the fibre core exciting unit (14) enters into the other end surface. The seed laser beam is amplified in the activated core during the propagation through the amp fibre (10) and comes out as a high power processing laser beam from the other end surface of the amp fibre (10). The light sensor (64) feeds back the laser power of the processing laser beam to the seed laser oscillating unit (12).