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 to obtain a laminate; providing at least one of surfaces of the laminate with a protection wafer having a thermal expansion coefficient smaller than that of the oxide single-crystal wafer; and heat-treating the laminate provided with the protection wafer at a temperature of 80°C or higher to split the laminate along the ion-implanted layer to obtain an oxide single-crystal film transferred onto the support wafer; 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 ; the support wafer is selected from the group consisting of a sapphire wafer, a silicon wafer, a silicon wafer with an oxide film, and a glass wafer; and the temperature of 80°C or higher in the step of heat-treating is from 80 to 150°C in a case where the support wafer is the sapphire wafer; from 80 to 125°C in a case where the support wafer is the silicon wafer or a silicon wafer with an oxide film; and from 80 to 110°C in a case where the support wafer is the glass wafer. 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 to obtain a laminate; providing at least one of surfaces of the laminate with a protection wafer having a thermal expansion coefficient smaller than that of the oxide single-crystal wafer; and heat-treating the laminate provided with the protection wafer at a temperature of 80°C or higher to split the laminate along the ion-implanted layer to obtain an oxide single-crystal film transferred onto the support wafer; 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 ; the support wafer is selected from the group consisting of a sapphire wafer, a silicon wafer, a silicon wafer with an oxide film, and a glass wafer; and the temperature of 80°C or higher in the step of heat-treating is from 80 to 150°C in a case where the support wafer is the sapphire wafer; from 80 to 125°C in a case where the support wafer is the silicon wafer or a silicon wafer with an oxide film; and from 80 to 110°C in a case where the support wafer is the glass wafer. <EOS>