A production method for a stereoscopic-image-forming device characterized by comprising: a first step of producing a molded preform made from a first transparent resin by injection-molding, the molded preform including triangle-cross-section first and second grooves (42, 43) formed respectively on both sides of a transparent plate material, the molded preform including triangle-cross-section first and second protruded strips (44, 45) formed respectively on the both sides of the transparent plate material, the first and second grooves (42, 43) each having a vertical surface (46, 48) and an inclined surface (47, 49) with an angle (θ1) therebetween of 15 to 60 degrees, the first protruded strips being formed by the first grooves next to each other, the second protruded strips being formed by the second grooves next to each other, the first and second grooves (42, 43) formed respectively on the both sides of the transparent plate material being arranged so as to be orthogonally crossed in a plan view; a second step of forming mirror surfaces (51, 52) selectively on the vertical surfaces (46, 48) of the first and second grooves (42, 43) provided on the both sides of the molded preform; and a third step of filling up a second transparent resin (53, 54) into the first and second grooves (42, 43) after performing the second step, and further applying a flattering treatment to the surface of the filled second transparent resin (53, 54), wherein in the second step the mirror surfaces (51, 52) are formed by sputtering, metal deposition, metal microparticle spraying, or ion beam irradiation toward the vertical surfaces (46, 48) from a direction along the inclined surfaces (47, 49) in a manner where the inclined surfaces (47, 49) become in shadow, and wherein a refractive index η2 of the second transparent resin (53, 54) is within a range of 0.95 to 1.05 times a refractive index η1 of the first transparent resin. a first step of producing a molded preform made from a first transparent resin by injection-molding, the molded preform including triangle-cross-section first and second grooves (42, 43) formed respectively on both sides of a transparent plate material, the molded preform including triangle-cross-section first and second protruded strips (44, 45) formed respectively on the both sides of the transparent plate material, the first and second grooves (42, 43) each having a vertical surface (46, 48) and an inclined surface (47, 49) with an angle (θ1) therebetween of 15 to 60 degrees, the first protruded strips being formed by the first grooves next to each other, the second protruded strips being formed by the second grooves next to each other, the first and second grooves (42, 43) formed respectively on the both sides of the transparent plate material being arranged so as to be orthogonally crossed in a plan view; a second step of forming mirror surfaces (51, 52) selectively on the vertical surfaces (46, 48) of the first and second grooves (42, 43) provided on the both sides of the molded preform; and a third step of filling up a second transparent resin (53, 54) into the first and second grooves (42, 43) after performing the second step, and further applying a flattering treatment to the surface of the filled second transparent resin (53, 54), wherein in the second step the mirror surfaces (51, 52) are formed by sputtering, metal deposition, metal microparticle spraying, or ion beam irradiation toward the vertical surfaces (46, 48) from a direction along the inclined surfaces (47, 49) in a manner where the inclined surfaces (47, 49) become in shadow, and wherein a refractive index η2 of the second transparent resin (53, 54) is within a range of 0.95 to 1.05 times a refractive index η1 of the first transparent resin.