The present invention provides a method for manufacturing a composite of a carbon nanomaterial and a metallic material which has a homogeneous composite metal structure and thixotropic properties by compositing a metallic material of a non-ferrous metal alloy with a carbon nanomaterial by using both stirring and ultrasonic vibration. The method comprises compositing the metallic material of the non-ferrous metal alloy with the carbon nanomaterial by adding the carbon nanomaterial in a state where the metallic material shows thixotropic properties by spheroidization of solid phase in a semi-solid state, and the compositing is performed by a process for stirring and kneading the semi-solid metallic material while keeping the temperature thereof at a solid-liquid coexisting temperature, and a process for dispersing the carbon nanomaterial to liquid phase between solid phases by ultrasonic vibration.