A method for manufacturing a composite piezoelectric substrate comprising: a first step of preparing a piezoelectric substrate and a supporting substrate; a second step of implanting ions from a surface of the piezoelectric substrate to form a defective layer in a region having a predetermined depth from the surface of the piezoelectric substrate; a third step of bonding the supporting substrate to the surface of the piezoelectric substrate to form a substrate bonded body; a fourth step of separating the substrate bonded body at the defective layer formed in the piezoelectric substrate so that a separation layer between the surface of the piezoelectric substrate and the defective layer is separated from the piezoelectric substrate and bonded to the supporting substrate to form a composite piezoelectric substrate; and a fifth step of smoothing the surface of the separation layer of the composite piezoelectric substrate; characterized in that there is further provided: a cleaning activation step of removing impurities adhering to at least one of the surface of the piezoelectric substrate in which the defective layer is formed and a surface of the supporting substrate to directly expose constituent atoms of the surface and activate it; and a polarization step of polarizing the separation layer of the composite piezoelectric substrate after the second step and before the cleaning activation step or after the cleaning activation step; and in that the third step bonds the supporting substrate to the surface of the piezoelectric substrate after the cleaning activation step. a first step of preparing a piezoelectric substrate and a supporting substrate; a second step of implanting ions from a surface of the piezoelectric substrate to form a defective layer in a region having a predetermined depth from the surface of the piezoelectric substrate; a third step of bonding the supporting substrate to the surface of the piezoelectric substrate to form a substrate bonded body; a fourth step of separating the substrate bonded body at the defective layer formed in the piezoelectric substrate so that a separation layer between the surface of the piezoelectric substrate and the defective layer is separated from the piezoelectric substrate and bonded to the supporting substrate to form a composite piezoelectric substrate; and a fifth step of smoothing the surface of the separation layer of the composite piezoelectric substrate; characterized in that there is further provided: a cleaning activation step of removing impurities adhering to at least one of the surface of the piezoelectric substrate in which the defective layer is formed and a surface of the supporting substrate to directly expose constituent atoms of the surface and activate it; and a polarization step of polarizing the separation layer of the composite piezoelectric substrate after the second step and before the cleaning activation step or after the cleaning activation step; and in that the third step bonds the supporting substrate to the surface of the piezoelectric substrate after the cleaning activation step. a cleaning activation step of removing impurities adhering to at least one of the surface of the piezoelectric substrate in which the defective layer is formed and a surface of the supporting substrate to directly expose constituent atoms of the surface and activate it; and a polarization step of polarizing the separation layer of the composite piezoelectric substrate after the second step and before the cleaning activation step or after the cleaning activation step; and in that the third step bonds the supporting substrate to the surface of the piezoelectric substrate after the cleaning activation step.