A composite-substrate manufacturing method is provided with: a step of carrying out implantation of ions through a surface of a bulk substrate composed of the nitride compound semiconductor; a step of setting said surface of the bulk substrate against the second substrate, and bonding the bulk substrate and the second substrate together to obtain a bonded substrate; a step of elevating the temperature of the bonded substrate to a first temperature; a step of sustaining the first temperature for a fixed time; and a step of producing a composite substrate by severing the remaining portion of the bulk substrate from the bonded substrate; characterized in that a predetermined formula as for the first temperature, the thermal expansion coefficient of the first substrate, and the thermal expansion coefficient of the second substrate is satisfied.