A method for preparing a bonded SOS substrate comprising a semiconductor film on a surface of a sapphire substrate, comprising the steps of: providing the sapphire substrate and a semiconductor substrate; implanting ions from a surface of the semiconductor substrate to form an ion-implanted layer; activating at least a surface selected from the surface of the sapphire substrate and the surface of the semiconductor substrate from which the ions have been implanted; bonding the surface of the semiconductor substrate and the surface of the sapphire substrate at a temperature of from 50°C to 350°C; heating the bonded substrates at a maximum temperature of from 200°C to 350°C to form a bonded body; irradiating visible light from a sapphire substrate side or a semiconductor substrate side to the ion-implanted layer of the semiconductor substrate for making an interface of the ion-implanted layer brittle, while keeping a temperature of the bonded body higher than the temperature at which the surfaces of the semiconductor substrate and the sapphire substrate were bonded, so as to transfer the semiconductor film to the sapphire substrate wherein the temperature of the bonded body during irradiation of the visible light is from 30°C to 100°C higher than the temperature for said bonding at which the bonded body was formed; and chemically etching and/or polishing a surface of the transferred semiconductor film, wherein the semiconductor substrate is a single crystal silicon substrate or a single crystal silicon substrate on which an oxide film has been grown, and the silicon on sapphire substrate comprises a single crystal silicon film having a thickness of 100 nm or less on or above the sapphire substrate, and a defect density of a surface of the single crystal silicon film measured by a Secco defect detection method and a selection etching defect detection method is 10 4 pieces/cm 2 or less. providing the sapphire substrate and a semiconductor substrate; implanting ions from a surface of the semiconductor substrate to form an ion-implanted layer; activating at least a surface selected from the surface of the sapphire substrate and the surface of the semiconductor substrate from which the ions have been implanted; bonding the surface of the semiconductor substrate and the surface of the sapphire substrate at a temperature of from 50°C to 350°C; heating the bonded substrates at a maximum temperature of from 200°C to 350°C to form a bonded body; irradiating visible light from a sapphire substrate side or a semiconductor substrate side to the ion-implanted layer of the semiconductor substrate for making an interface of the ion-implanted layer brittle, while keeping a temperature of the bonded body higher than the temperature at which the surfaces of the semiconductor substrate and the sapphire substrate were bonded, so as to transfer the semiconductor film to the sapphire substrate wherein the temperature of the bonded body during irradiation of the visible light is from 30°C to 100°C higher than the temperature for said bonding at which the bonded body was formed; and chemically etching and/or polishing a surface of the transferred semiconductor film, wherein the semiconductor substrate is a single crystal silicon substrate or a single crystal silicon substrate on which an oxide film has been grown, and the silicon on sapphire substrate comprises a single crystal silicon film having a thickness of 100 nm or less on or above the sapphire substrate, and a defect density of a surface of the single crystal silicon film measured by a Secco defect detection method and a selection etching defect detection method is 10 4 pieces/cm 2 or less.