A method of producing a composite wafer having an oxide single-crystal film on a support wafer, comprising steps of: implanting hydrogen atom ions or hydrogen molecule ions into an oxide single-crystal wafer through a surface thereof, which wafer is a lithium tantalate or lithium niobate wafer, to form an ion-implanted layer inside the oxide single-crystal wafer; subjecting at least one of the surface of the oxide single-crystal wafer and a surface of a support wafer to be laminated with the oxide single-crystal wafer to surface activation treatment; after the surface activation treatment, bonding the surface of the oxide single-crystal wafer to the surface of the support wafer at a temperature of from 10 to 50°C to obtain a laminate; and heat-treating the laminate at a temperature of 90°C or higher at which cracking is not caused; wherein an implantation dose of the hydrogen atom ions is from 5.0×10 16 atom/cm 2 to 2.75×10 17 atom/cm 2 and an implantation dose of the hydrogen molecule ions is from 2.5×10 16 atoms/cm 2 to 1.37×10 17 atoms/cm 2, characterised in that the method further comprises: the step of applying ultrasonic vibration to the heat-treated laminate to split the laminate along the ion-implanted layer to obtain an oxide single-crystal film transferred onto the support wafer; and further characterised in that: the step of heat-treating the laminate is performed at a temperature at which cracking is not caused and a difference between a temperature at the bonding to obtain the laminate and a temperature of the laminate during the application of the ultrasonic vibration is from 0 to 40°C. implanting hydrogen atom ions or hydrogen molecule ions into an oxide single-crystal wafer through a surface thereof, which wafer is a lithium tantalate or lithium niobate wafer, to form an ion-implanted layer inside the oxide single-crystal wafer; subjecting at least one of the surface of the oxide single-crystal wafer and a surface of a support wafer to be laminated with the oxide single-crystal wafer to surface activation treatment; after the surface activation treatment, bonding the surface of the oxide single-crystal wafer to the surface of the support wafer at a temperature of from 10 to 50°C to obtain a laminate; and heat-treating the laminate at a temperature of 90°C or higher at which cracking is not caused; wherein an implantation dose of the hydrogen atom ions is from 5.0×10 16 atom/cm 2 to 2.75×10 17 atom/cm 2 and an implantation dose of the hydrogen molecule ions is from 2.5×10 16 atoms/cm 2 to 1.37×10 17 atoms/cm 2, characterised in that the method further comprises: the step of applying ultrasonic vibration to the heat-treated laminate to split the laminate along the ion-implanted layer to obtain an oxide single-crystal film transferred onto the support wafer; and further characterised in that: the step of heat-treating the laminate is performed at a temperature at which cracking is not caused and a difference between a temperature at the bonding to obtain the laminate and a temperature of the laminate during the application of the ultrasonic vibration is from 0 to 40°C. the step of applying ultrasonic vibration to the heat-treated laminate to split the laminate along the ion-implanted layer to obtain an oxide single-crystal film transferred onto the support wafer; and further characterised in that: the step of heat-treating the laminate is performed at a temperature at which cracking is not caused and a difference between a temperature at the bonding to obtain the laminate and a temperature of the laminate during the application of the ultrasonic vibration is from 0 to 40°C. the step of heat-treating the laminate is performed at a temperature at which cracking is not caused and a difference between a temperature at the bonding to obtain the laminate and a temperature of the laminate during the application of the ultrasonic vibration is from 0 to 40°C.