PURPOSE: To provide the wavelength conversion element with which the high intensity of a second harmonic wave is obtainable with an increase in the intensity of an incident basic wave by adding a specific heat conduction layer onto a KTiOPO4 substrate.

CONSTITUTION: The heat conduction layer 3 having ≥3W/m°C thermal conductivity and ≥90% light transmittance at 350 to 200nm wavelength is added onto the KTiOPO4 substrate 1. The lens light emitted from a titanium sapphire laser 4 of 860nm wavelength is condensed to the end face of an optical wavelength 2 by a lens 5 and is guided as a basic wave 6 in the optical wavelength 2 with a wavelength conversion device constituted by using this wavelength conversion element. The basic wave 6 propagating in the optical wavelength 2 is converted by the quadratic nonlinear effect of the optical wavelength 2 to a second harmonic wave 7 of 430nm wavelength propagating in the optical wavelength 2. The temp. rise in the optical wavelength 2 by the incident light of high intensity is prevented by providing the element with such heat conduction layer 3, by which the refractive index conversion of the optical wavelength 2 part is suppressed. The attenuation of the light is simultaneously prevented by making the light conduction layer 3 transparent.