PURPOSE: To make it possible to obtain high wavelength conversion efficiency by forming a domain inversion region over a part of the overall length of the optical wavelength conversion element, thereby surely compensating the phase difference or walk off of a basic wave by a crystal, enabling the formation at a relatively low cost and suppressing the loss by the element to a low level.

CONSTITUTION: The KTP crystal 10 is formed with the domain inversion region 10B over a half of the overall length in its light passage direction. The phase difference of a laser beam 11 generated by a non-inversion region 10A and the phase difference of the laser beam 11 generated by the domain inversion region 11B are opposite to each other in directions and negate each other as their absolute values are equaled to each other when the laser beam 11 as the basic wave is made incident on the KTP crystal 10 of this constitution. The angle formed by the linear polarization direction of the laser beam 11 with the Z-axis of the KTP crystal 10 is kept at 45° and second higher harmonic waves 12 are eventually generated with the max. efficiency.