A method of supplying cooling water to a laser processing head includes arranging a cooling water supply path connected to the laser processing head, connecting, to the cooling water supply path, a water supply path for supplying water and an antirust supply path for supplying antirust, keeping constant a ratio between a water quantity fed by a water feed means connected to the water supply path and an antirust quantity fed by an antirust feed means connected to the antirust supply path, mixing the water and antirust in the cooling water supply path, and supplying cooling water to the laser processing head.