In a method for manufacturing a composite-metal-forming material, heating a metal material a Mg alloy or an Al alloy is heated to a temperature in a region where a solid and a liquid are both present to thereby yield a semi-molten metal material in a semi-molten state. An additive material is introduced to the semi-molten metal material and kneading is performed to obtain a composite metal material. The composite metal material is heated to a solution temperature of the metal material and a solution treatment is performed to thereby yield a composite-metal-forming material. The additive material introduced is a carbon nano-composite material formed by mixing a carbon nanomaterial and a metal powder to obtain a carbon nano-composite metal powder, compacting the carbon nano-composite metal powder into a solid to obtain a preform, heating the preform in a vacuum, inert gas, or non-oxidizing gas atmosphere to a temperature in a region where a solid and a liquid are both present, and applying pressure to the heated preform.