A laser processing system herein includes a laser oscillator, a laser optical path that guides laser beam from the laser oscillator to a workpiece, a purge gas supply line for supplying a purge gas into the laser optical path, oxygen sensor and an impure gas sensor which detects an impure gas influencing the propagation of the laser beam that are installed in the laser optical path, and an impure gas sensor output value correction unit. The impure gas sensor output value correction unit corrects an output value of the impure gas sensor based on an output value of the oxygen sensor.