In an optical wavelength converting method in which light from a laser oscillator that oscillates coherent light of an inherent wavelength .lamda. is employed as incident light, and is made to input to a nonlinear optical crystal, and light having a wavelength of 1/2.lamda. is radiated, the wavelength of the incident light is 1000 nm or less, and the peak power density of the incident light is 0.1 10 fold greater than the peak power density that provides the maximum conversion efficiency. The nonlinear optical crystal is heated to and maintained at 200 600.degree. C. Prescribed fundamental waves are input to a first crystal and a second crystal sequentially. The first crystal has a higher bulk damage threshold value with respect to the fundamental waves than that of the second crystal. The second crystal has a higher effective nonlinear constant with respect to the fundamental waves than that of the first crystal.