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PROBLEM TO BE SOLVED : To provide a heat treatment method of a bismuth-substituted rare earth iron garnet single crystal that enables a heat treatment in a short time without etching (reducing) a surface of a RIG membrane.SOLUTION : A heat treatment method of a bismuth-substituted rare earth iron garnet single crystal grown on a non-magnetic garnet substrate using a melt containing a raw material component of rare earth iron garnet single crystal, a flux component containing lead oxide and a Ca component having a Pb content of less than 0.1 wt% and applied to a Faraday rotator for an optical isolator of 1 μm wavelength band, in which the surface roughness (Ra) of the bismuth-substituted rare earth iron garnet single crystal to be heat treated is 0.2 to 0.4 μm, an atmosphere gas during the heat treatment consists of 45 to 60 volume% of hydrogen gas and a residual part of nitrogen gas, and the temperature of the heat treatment is 300 to 400°C.