PURPOSE: To perform satisfactory working by providing a magnetic sensor or electrostatic capacity type sensor in the end part of a nozzle for blowing an assist gas, and measuring the distance between the working head and the work in the part where the assist gas is blown and the part where a laser is irradiated.

CONSTITUTION: A titled head is constituted of a noncontact sensor 16, a nozzle 2 constituted of a water cooling part 17 for cooling the same, a working head 1 for holding the same, and a head driving part which moves upward or downward apart at a gap (g) therefrom. If the sensor 16 is a magnetic sensor, the change ΔZ of the impedance Z in the sensor coil is ΔZ∝g with g designated as a gap, and in the case of an electrostatic type sensor, the change ΔC in the electrostatic capacity C is ΔC∝1/g. Therefore, the g can be calculated by detecting ΔZ, ΔC. Since the gap in the working position is measured in such a way, the accuracy is made several times higher than in the prior art.