Abstract: A second harmonic wave generating device comprises a LiTaO.sub.3 thin film formed on a LiNbO.sub.3 substrate and a LiNbO.sub.3 thin film waveguide layer formed in the LiTaO.sub.3 thin film. The thickness (T(.mu.m)) of the LiNbO.sub.3 thin film waveguide layer, an ordinary refractive index (.eta..sub.oS1) of the LiTaO.sub.3 thin film at a fundamental wavelength (.lambda.(.mu.m)), an ordinary refractive index (.eta..sub.oF1) of the LiNbO.sub.3 thin film waveguide layer at the fundamental wavelength (.lambda.(.mu.m)), an extraordinary refractive index (.eta..sub.eS2) of the LiTaO.sub.3 thin film at a second harmonic wavelength (.lambda./2, (.mu.m)) and an extraordinary refractive index (.eta..sub.eF2) of the LiNbO.sub.3 thin film waveguide layer at the second harmonic wavelength (.lambda./2, (.mu.m)) are individually within the following specific values:.lambda.=0.68 to 0.94 .mu.mT=0.1 to 20 .mu.m.eta..sub.oF1 =2.22 to 2.38.eta..sub.oS1 =2.10 to 2.25.eta..sub.eF2 =2.24 to 2.42.eta..sub.eS2 =2.22 to 2.38.