An ultraviolet laser is provided that increases the conversion efficiency
of laser beams when harmonics having wavelengths of less than 355 nm are
generated by converting laser beams with a wavelength conversion element
made of a single crystal of lithium tetraborate. A wavelength conversion
element is heated and controlled to a specific temperature (for example,
200.degree. C.) within a temperature range of 200 to 450.degree. C. with
a predetermined temperature precision.