A laser machining device includes a reflective plate disposed perpendicular to the optical axis of emitting light and having a constant reflectance to the emitting light; a return light measurement unit which measures intensity distribution of return light reflected off the reflective plate and returning to the external optical system via a beam splitter; a storage unit which stores the return light intensity distribution in a normal state as reference data; a preprocessing unit which performs processing of identifying at least one of an optical axis shift, a beam diameter anomaly, a mode anomaly, a ghost, contamination of a protective window, and a focus shift due to thermal lens effect on the basis of comparison between measurement data of the return light intensity distribution and the reference data, before laser machining; and a warning unit which warns of an anomaly in the external optical system in accordance with the preprocessing unit.