A laser processing device comprising: a laser oscillation unit configured to oscillate and output a laser beam (LB) for laser processing; a processing head optically connected to the laser oscillation unit via an optical fiber cable, and configured to focally irradiate a processing point of a workpiece (W) with the laser beam (LB) from the laser oscillation unit; and a laser processing monitoring device for photoelectrically converting a predetermined beam to be measured by an optical sensor disposed in the processing head or in proximity to the processing head to acquire a sensor output signal representing light intensity of the beam to be measured, and monitoring the laser processing based on the sensor output signal, during irradiation to the workpiece (W) with the laser beam (LB) for laser processing by the processing head , the predetermined beam to be measured being generated or reflected in a vicinity of a processing point of the workpiece (W), and being characterised by : the laser processing monitoring device comprising: a reference beam source provided in the processing head , and configured to generate a reference beam for calibrating the optical sensor ; a reference beam source power supply unit configured to supply, to the reference beam source , adjustable power for generating the reference beam; and an optical measuring instrument having a beam receiver for receiving the reference beam from the reference beam source in order to calibrate the reference beam source , and configured to measure light intensity of the received reference beam or a predetermined physical quantity equivalent to the light intensity. a laser oscillation unit configured to oscillate and output a laser beam (LB) for laser processing; a processing head optically connected to the laser oscillation unit via an optical fiber cable, and configured to focally irradiate a processing point of a workpiece (W) with the laser beam (LB) from the laser oscillation unit; and a laser processing monitoring device for photoelectrically converting a predetermined beam to be measured by an optical sensor disposed in the processing head or in proximity to the processing head to acquire a sensor output signal representing light intensity of the beam to be measured, and monitoring the laser processing based on the sensor output signal, during irradiation to the workpiece (W) with the laser beam (LB) for laser processing by the processing head , the predetermined beam to be measured being generated or reflected in a vicinity of a processing point of the workpiece (W), and being characterised by : the laser processing monitoring device comprising: a reference beam source provided in the processing head , and configured to generate a reference beam for calibrating the optical sensor ; a reference beam source power supply unit configured to supply, to the reference beam source , adjustable power for generating the reference beam; and an optical measuring instrument having a beam receiver for receiving the reference beam from the reference beam source in order to calibrate the reference beam source , and configured to measure light intensity of the received reference beam or a predetermined physical quantity equivalent to the light intensity. a reference beam source provided in the processing head , and configured to generate a reference beam for calibrating the optical sensor ; a reference beam source power supply unit configured to supply, to the reference beam source , adjustable power for generating the reference beam; and an optical measuring instrument having a beam receiver for receiving the reference beam from the reference beam source in order to calibrate the reference beam source , and configured to measure light intensity of the received reference beam or a predetermined physical quantity equivalent to the light intensity. <EOS>