a process for producing a second harmonic wave generating device which converts the wavelength of the incident laser beam to 1 / 2 wavelength thereof by utilizing the non - linear optical effect possessed by an optical crystal . in forming a light waveguide according to the angle phase - matching method , the desired light waveguide is formed by generating practically the second harmonic wave and therefore phase matching can be effected very easily , and also its matching precision is higher , whereby a second harmonic wave generating device with excellent shg conversion efficiency can be produced .