To provide a laser beam machining device provided with a laser beam monitoring means for machining, which can guarantee reliability even for use over a long period of time, further, can effectively discriminate not only the disconnection of an optical fiber propagating a laser beam for machining but also wear or the like of an optical element incorporated into a laser beam generating source.

The laser beam monitoring means for machining comprises: an optical fiber for beam return for returning a part of a laser beam for machining propagated to the head part for machining to the device body part; and a return beam sensor which is located at the device body part, photoelectrically converts a return beam emitted from the optical fiber for beam return, and detects the same.

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