A bismuth-substituted rare-earth iron garnet crystal film (RIG) which has an insertion loss of less than 0.6 dB and which can be produced in a high yield, as well as an optical isolator, which is grown by liquid phase epitaxy on a non-magnetic garnet substrate represented by a chemical formula of Gd3(ScGa)5O12, wherein the RIG is represented by a chemical formula of Nd3-x-yGdxBiyFe5O12, and x and y satisfy 0.89≦x≦1.43 and 0.85≦y≦1.19. In contrast to conventional RIGs, the RIG represented by the chemical formula of Nd3-x-yGdxBiyFe5O12 of the present invention has an insertion loss of less than 0.6 dB and makes it possible to reduce the amount of heat generated because of absorption of light at wavelengths of about 1 μm. Hence, the RIG has such a remarkable effect that the RIG can be used as a Faraday rotator used for an optical isolator in a high-power laser device for processing.