A KTP crystal is irradiated by a long-pulse fundamental wave pulse laser beam having a pulse width of 100 .mu.s or longer typically output from a solid-state pulse laser such as a YAG pulse laser. In the KTP crystal, wavelength conversion is performed to generate a second harmonic pulse laser beam with a two times higher frequency, which long-pulse second harmonic pulse laser beam is in turn output for laser processing such as welding etc.