An optical wavelength converting apparatus comprises an optical wavelength converting device, which is constituted of a crystal of a nonlinear optical material and converts a wavelength of an incident fundamental wave into a different wavelength, a holder, which is constituted of a metal and to which the optical wavelength converting device is adhered and secured, and a device for adjusting the temperature of the region containing the optical wavelength converting device. The optical wavelength converting device is secured to the holder by adhering only the surface of the optical wavelength converting device to the holder, which surface is normal to the direction of a crystallographic axis that has a coefficient of thermal expansion most different from the coefficient of thermal expansion of the metal constituting the holder among the coefficients of thermal expansion of the crystallographic axes of the optical wavelength converting device. The optical wavelength converting device is thus reliably secured to the holder and prevented from breaking or being distorted due to a change in environmental temperature, and the accuracy, with which the temperature of the optical wavelength converting device is adjusted, is kept high.