A nozzle holding means having a nozzle being free to eject laser beam from its top end is provided being free to move along a guide means among a plurality of machine tools. A voltage detector is provided for detecting the voltage across the switching device to produce a detection voltage which is a function of the voltage across the switching device, and circuit arrangement is used to predict a valley timing for the voltage across the switching device by evaluating the time period that the detection voltage falls down from a first threshold to a second threshold. Each first optical fiber has a positive chromatic dispersion at a signal light wavelength while each second optical fiber has a negative chromatic dispersion at the same wavelength.