In a laser machining apparatus that performs machining by a laser beam (4) emitted from a laser oscillator (1), the laser machining apparatus herein provided includes an aperture (5) placed in a light path of the laser beam (4) emitted from the laser oscillator (1) so as to block a perimeter portion of the laser beam (4) and to transmit a middle portion thereof, and a beam-power measurement sensor (6) for measuring beam power of a laser beam (20) transmitted through the aperture (5), whereby it utilizes that beam power of the laser beam transmitted through the aperture (5) significantly changes (the more degraded, the more the beam power rises) due to a degradation condition of an output mirror (2) when the output mirror in the laser oscillator (1) becomes in high thermal loading condition due to the laser beam with high beam power, so that the degradation condition of the output mirror (2) is determined.