PURPOSE: To improve the inertia characteristics by providing a harmonic drive reduction gear which has a hole part for the laser beam path at the central part and a large reduction gear ratio.

CONSTITUTION: A harmonic drive reduction gear 8 which keeps the gear head rotationably, reduces the velocity of rotation of front arms 2, 3 containing the laser beam path of the laser head in it and driving motors 5, 6, rotates either of both and is a large reduction gear ratio and has a hole part which becomes a part of the laser beam path at the central part is provided in the driving device of the laser beam head. In such a way, the laser beam driving device which has excellent characteristic of the torque, the inertia, etc., and can be applied for comparative large laser energy can be obtained.