A method of irradiating ultraviolet light onto an object. The method includes generating pulse light having a single wavelength within a range from 1.51 .mu.m to 1.59 .mu.m from a laser generating portion, amplifying the pulse light plural times with an optical amplifier that includes a plurality of fiber optical amplifiers, wavelength-converting the amplified pulse light into ultraviolet light with a wavelength converting portion that includes a plurality of non-linear optical crystals, and irradiating the ultraviolet light onto the object through an optical system.