in the following , examples of the present invention are described in detail by referring to the accompanying drawing . a ti thin film formed on an x cut linbo 3 single crystal substrate 1 with a thickness of 0 . 5 mm ( fig1 ( a )) was thermally diffused to prepare an optical circuit device having a slab type light waveguide comprising linbo 3 having diffused ti therein ( fig1 ( b )). that is , on the above substrate 1 , a ti thin film with a thickness of 400 a was formed according to the high frequency ( rf ) sputtering method , and the ti thin film was thermally diffused in a humidified ar gas stream at 1000 ° c . for 5 hours and further in a humidified 0 2 gas stream at 1000 ° c . for one hour to form a light waveguide 2 . on the light waveguide 2 of the above optical circuit device , a visible light light - sensitive photoresist was coated to a thickness of 2 μm as the visible light light - sensitive material 3 ( fig1 ( c )). after mirror polishing of the light incident and exit side end surfaces of the optical circuit device by vibrating buff polishing , it was placed on a sample stage which was made horizontally rotatable around the vertical axis , and a he - ne laser beam 4 with a wavelength of 1 . 15 μm was permitted to be incident into the light waveguide 2 . and , phase matching was effected by rotating the above sample stand a fine angle ( fig1 ( d )). by such phase matching , the second harmonic wave with a wavelength of 0 . 575 μm was generated and the light - sensitive material 3 coated on the surface of the light waveguide 2 was exposed to the same pattern as the second harmonic wave by the scattered light generated during the propagation of the second harmonic wave through the light waveguide 2 . subsequently , the light - sensitive material 3 was developed to provide an etching mask 5 corresponding to the route of the second harmonic wave ( fig1 ( e )). the step of coating the visible light light - sensitive material 3 on the light waveguide 2 to the step of developing the light - sensitive material 3 were worked in a dark room or red safe light to which the light - sensitive material 3 was not sensitive . next , an etching treatment was applied on the above light waveguide 2 by ion beam etching to effect etching of the portion other than the etching mask 5 . and , the etching mask 5 was removed from the optical circuit device to prepare a shg device as shown in fig1 ( f ). when the shg conversion efficiency of the shg device was measured , when a he - ne laser with a wavelength of 1 . 15 μm and 1 mw was used as the light source , it was found to be 1 × 10 - 4 , which was higher as compared with the device of the prior art having an efficiency of 2 × 10 - 5 . first , an optical circuit device having the light waveguide 2 formed on the substrate 1 was prepared in the same manner as in example 1 . subsequently , a photographic light - sensitive emulsion ( liquid light ) was coated on the light waveguide 2 to a thickness of 0 . 5 μm as the visible light light - sensitive material 3 . and , after the light incident and exit side end surfaces were mirror worked similarly as in example 1 , a he - ne laser beam 4 with a wavelength of 1 . 15 μm wa permitted to be incident into the light waveguide 2 to effect phase matching . with such phase matching , the second harmonic wave with a wavelength of 0 . 575 μm was generated , and the lightsensitive material 3 was exposed to the same pattern as the route of the second harmonic wave generated . subsequently , the light - sensitive material 3 was developed to discolor the light - sensitive portion 6 corresponding to the rote of the second harmonic wave ( fig2 ( a )). on the light - sensitive material 3 , a positive - type photoresist 7 was coated by a spin coater ( fig2 ( b )). the rotational number of the spin coater was 1800 rpm , and the coating film thickness was 2 μm . and , the photographic sensitive plate photomask 8 applied with a linear pattern with a width of 10 μm was contacted on the resist 7 by mask aligning with the use of a mask aligner so as to corresponding to the light - sensitive portion 6 of the above light - sensitive material 3 seen through the photo - resist 7 ( fig2 ( c )), and the resist 7 was exposed with uv - ray and developed to obtain a resist pattern for the etching mask 9 ( fig2 ( d )). the etching mask 9 was found to have a width of 10 μm and a thickness of 2 μm . subsequently , ion beam etching ( acceleration voltage 2 kv , ion current density 0 . 9 ma / cm 2 ) was applied on the light waveguide 2 and the light - sensitive material 3 to etch the portion other than the etching mask 9 to obtain a channel type waveguide 2 with a width of 10 μm and a thickness of 2 μm ( fig2 ( e )). finally , the light - sensitive material 3 and the etching mask 9 were removed with a solvent ( fig2 ( f )). when the conversion efficiency of the shg device obtained was measured , when a he - ne laser with a wavelength of 1 . 152 μm and 5 mw was used as the light source , it was found to be 4 × 10 - 4 , which was higher as compared with the device of the prior art having an efficiency of 2 × 10 - 5 . the steps before resist coating were the same as in example 2 , but a negative - type photoresist 7 was coated on the light - sensitive material 3 by a spin coater ( fig3 ( a ), ( b )). the rotational number of the spin coater was 3600 rpm , and the coating film thickness was 1 μm . subsequently , mask alignment was effected in the same manner as in example 2 ( fig3 ( c )), followed by exposure and developing , to obtain a resist pattern 10 ( fig3 ( d )). next , a ti film 11 was formed to a thickness of 1 μm by rf sputtering ( fig3 ( e )). and , according to the lift - off method in which the resist pattern 10 and the ti film 11 on said resist pattern 10 were removed with a solvent , a ti pattern for the etching mask 12 with a width of 10 μm was obtained . and , ion beam etching ( acceleration voltage 2 kv , current density 0 . 9 ma / cm 2 ) was applied on the light waveguide 2 and the light - sensitive material 3 for 3 hours to etch the portion other than the etching mask 12 , thereby obtaining a channel type waveguide 2 with a width of 10 μm and a thickness of 2 μm ( fig3 ( g )). finally , the light - sensitive material 3 and the etching mask 12 were removed with a solvent ( fig3 ( h )). when the conversion efficiency of the shg device obtained was measured , when a he - ne laser with a wavelength of 1 . 152 μm and 5 mw was used as the light source , it was found to be 4 × 10 - 4 , which was higher as compared with the device of the prior art having an efficiency of 2 × 10 - 5 . the steps to mirror finishing of the light incident and exit side end surfaces were the same as in example 2 , but as the laser beam source , a semiconductor laser beam with a wavelength of 0 . 83 μm was permitted to enter the light waveguide as a parallel light by a lens to effect phase matching . by such phase matching , the second harmonic wave with a wavelength of 0 . 415 μm was generated , and the light - sensitive material 3 was exposed to the same pattern as the route of the second harmonic wave generated . subsequently , the same steps as in example 2 were conducted , and the conversion efficiently of the shg device obtained was measured . as the result , when a semiconductor laser with a wavelength of 0 . 83 μm and 30 mw was used as the light source , it was found to be 1 × 10 - 2 , which was higher as compared with the device of the prior art having an efficiency of 1 × 10 - 3 . the steps were the same as in example 1 , but as the visible light light - sensitive material 3 , a conventional photoresist for uv - ray exposure having a light - sensitive region up to a wavelength of about 0 . 4 μm was used and a semiconductor laser with a wavelength of 0 . 83 μm as the laser light source to prepare a shg device . in this case , since the photo - resist is not sensitive to a wavelength of 0 . 5 μm or higher , and therefore the all steps were conducted under conventional yellow safe light without use of a dark room or red safe light . when the conversion efficiency of the shg device obtained was measured , when a semiconductor laser with a wavelength of 0 . 83 μm and 30 mw was used as the light source , it was found to be 2 × 10 - 2 , which was higher as compared with the device of the prior art having an efficiency of 1 × 10 - 3 . the present invention , as described above , forms the desired light waveguide by practically generating the second harmonic wave and therefore is extremely easy in phase matching , and also high in its matching precision , whereby a second harmonic wave generating device excellent in shg conversion efficiency can be produced .